The sea in the Quran is mentioned in relation to important information or similitude. It was not mentioned in a trivial or unimportant information like to say for example the sea so passes by the town so and so.
1- God honored us and carried us in the land and sea: he gave us the means to inhabit both form knowledge our brains are created to find or row materials needed for that.
17:70 Verily We have honored the children of Adam. We carry them on the land and the sea, and have made provision of good things for them, and have preferred them above many of those whom We created with a marked preferment. وَلَقَدْ كَرَّمْنَا بَنِي آدَمَ وَحَمَلْنَاهُمْ فِي الْبَرِّ وَالْبَحْرِ وَرَزَقْنَاهُم مِّنَ الطَّيِّبَاتِ وَفَضَّلْنَاهُمْ عَلَى كَثِيرٍ مِّمَّنْ خَلَقْنَا تَفْضِيلاً (17:70)
2- God provided us with all the fresh and soft meat and the ornaments we wear:
16:14 And He it is Who hath constrained the sea to be of service that ye eat fresh meat from thence, and bring forth from thence ornaments which ye wear. And thou seest the ships ploughing it that ye (mankind) may seek of His bounty, and that haply ye may give thanks. - وَهُوَ الَّذِي سَخَّرَ الْبَحْرَ لِتَأْكُلُواْ مِنْهُ لَحْمًا طَرِيًّا وَتَسْتَخْرِجُواْ مِنْهُ حِلْيَةً تَلْبَسُونَهَا وَتَرَى الْفُلْكَ مَوَاخِرَ فِيهِ وَلِتَبْتَغُواْ مِن فَضْلِهِ وَلَعَلَّكُمْ تَشْكُرُونَ (16:14)
3- There are salted seas and non salted seas or rivers:
25:53 And He it is Who hath given independence to the two seas (though they meet); one palatable, sweet, and the other saltish, bitter; and hath set a bar and a forbidding ban between them. وَهُوَ الَّذِي مَرَجَ الْبَحْرَيْنِ هَذَا عَذْبٌ فُرَاتٌ وَهَذَا مِلْحٌ أُجَاجٌ وَجَعَلَ بَيْنَهُمَا بَرْزَخًا وَحِجْرًا مَّحْجُورًا (
27:61 Is not He (best) Who made the earth a fixed abode, and placed rivers in the folds thereof, and placed firm hills therein, and hath set a barrier between the two seas? Is there any God beside Allah? Nay, but most of them know not! أَمَّن جَعَلَ الْأَرْضَ قَرَارًا وَجَعَلَ خِلَالَهَا أَنْهَارًا وَجَعَلَ لَهَا رَوَاسِيَ وَجَعَلَ بَيْنَ الْبَحْرَيْنِ حَاجِزًا أَإِلَهٌ مَّعَ اللَّهِ بَلْ أَكْثَرُهُمْ لَا يَعْلَمُونَ (27:61)
35:12 And two seas are not alike: this, fresh, sweet, food to drink, this (other) bitter, salt. And from them both ye eat fresh meat and derive the ornament that ye wear. And thou seest the ship cleaving them with its prow that ye may seek of His bounty, and that haply ye may give thanks. وَمَا يَسْتَوِي الْبَحْرَانِ هَذَا عَذْبٌ فُرَاتٌ سَائِغٌ شَرَابُهُ وَهَذَا مِلْحٌ أُجَاجٌ وَمِن كُلٍّ تَأْكُلُونَ لَحْمًا طَرِيًّا وَتَسْتَخْرِجُونَ حِلْيَةً تَلْبَسُونَهَا وَتَرَى الْفُلْكَ فِيهِ مَوَاخِرَ لِتَبْتَغُوا مِن فَضْلِهِ وَلَعَلَّكُمْ تَشْكُرُونَ (35
4- There is barrier between seas: A great phenomenon is the barrier between seas that God mentioned in the Quran as a sign to us that the words of the Quran is his.
55:19 He hath loosed the two seas. They meet. مَرَجَ الْبَحْرَيْنِ يَلْتَقِيَانِ (55:19)
55:20 There is a barrier between them. They encroach not (one upon the other). بَيْنَهُمَا بَرْزَخٌ لَّا يَبْغِيَانِ (55:20)
According to http://www.quranmiracles.com/articles.asp?id=47
A famous French oceanographer, J. Cousteau, gives the following account as a result of his studies in water barriers: “We studied the assertions by certain researchers about barriers separating seas, and noticed that the Mediterranean Sea had its own salinity and density and housed autochthonous fauna and flora. Then we examined the water of the Atlantic Ocean and discovered features entirely different from those of the Mediterranean Sea. According to our expectations, these two seas that merged in the strait of Gibraltar should present similar characteristics in terms of salinity, density and other properties. The two seas presented different features even though they were adjacent. This greatly puzzled us. An incredible barrier prevented the two seas from coming together. The same sort of a barrier had also been observed in Bab Al-Mandab in the Gulf of Aden connecting with the Red Sea. Subsequent to our observations, further researchers made it clear that the seas which had different characteristics had some barriers.” This fact that astounded oceanographers was revealed 14 centuries ago in the Quran. This aspect, not visible to the naked eye and appearing to be in conflict with the properties of water known to man, was first revealed to men in the Arabian peninsula, men who were not at all experienced in any sense with oceanography.
5- The water cycle is mentioned in the Quran*:
The Qur'an says in Surah Al-Hijr, Ch. No. 15, Verse No 22, it says that… ‘We send fecundating winds, winds impregnating, and cause rain to descend from the sky, and give you water in due measure’. The Arabic word 'Lawaaqi' used here, is the plural of 'laqi' derived from ‘laqaha’ which means ‘to impregnate or to fecundate’. The winds carrying the pollen - they impregnate the cloud and then rain falls. The wind causes the clouds to merge – there is condensation – there is lightning and rain falls from the clouds. The Qur'an describes the complete water cycle - how does the water evaporate, how it forms clouds, how it moves in the interior, how it falls down, how it flows back into the ocean in several places in great detail. It's mentioned in Surah Al Imran, Ch. No. 3, Verse No 9, in Surah Araf, Ch. No. 7, Verse No 57; In Surah Rad, Ch. No. 13, Verse No 17; In Surah Furqan, Ch. No. 25, Verse No 48 to 49; In Surah Fatir, Ch. No. 35, Verse No 9; In Surah Yasin, Ch. No. 36, Verse No 34; In Surah Jathiyah, Ch. No. 45, Verse No 5; In Surah Qaf, Ch. No. 50, Verse No 9 to 10; In Surah Waqiah, Ch. No. 56, Verse No 68 to 70; as well as in Surah Mulk, Ch. No. 67, Verse No 30. The Qur'an describes the Hydrology and Water cycle, in great detail.
6- The sea is mentioned 32 times and the land 13 see the coincidence:
Sea 32, Land 13
Sea + land = 32+13= 45
Sea = 32/45*100=71.11111111%
Land = 13/45*100 = 28.88888889%
Sea + land =100.00%
Modern science has only recently proven that the water covers 71.111% of the earth, while the land covers 28.889%.Is this a coincidence?
7- The sea mentioned in a great religious event the saving of the children of Israel:
2:50 And when We brought you through the sea and rescued you, and drowned the folk of Pharaoh in your sight. وَإِذْ فَرَقْنَا بِكُمُ الْبَحْرَ فَأَنجَيْنَاكُمْ وَأَغْرَقْنَا آلَ فِرْعَوْنَ وَأَنتُمْ تَنظُرُونَ (2:50)
10:90 And We brought the Children of Israel across the sea, and Pharaoh with his hosts pursued them in rebellion and transgression, till, when the (fate of) drowning overtook him, he exclaimed: I believe that there is no God save Him in whom the Children of Israel believe, and I am of those who surrender (unto Him). وَجَاوَزْنَا بِبَنِي إِسْرَائِيلَ الْبَحْرَ فَأَتْبَعَهُمْ فِرْعَوْنُ وَجُنُودُهُ بَغْيًا وَعَدْوًا حَتَّى إِذَا أَدْرَكَهُ الْغَرَقُ قَالَ آمَنتُ أَنَّهُ لا إِلِـهَ إِلاَّ الَّذِي آمَنَتْ بِهِ بَنُو إِسْرَائِيلَ وَأَنَاْ مِنَ الْمُسْلِمِينَ (10:90)
8- God save us from the distress of the sea: he give us an example of near drowning and how we plea to God to save us. The same can be applied in similar situations. The thing about this similitude is that Muhammad never ride a ship and this similitude attest to the Author of the Quran is God.10:22 He it is Who maketh you to go on the land and the sea till, when ye are in the ships and they sail with them with a fair breeze and they are glad therein, a storm wind reacheth them and the wave cometh unto them from every side and they deem that they are overwhelmed therein; (then) they cry unto Allah, making their faith pure for Him only; If Thou deliver us from this, we truly will be of the thankful. هُوَ الَّذِي يُسَيِّرُكُمْ فِي الْبَرِّ وَالْبَحْرِ حَتَّى إِذَا كُنتُمْ فِي الْفُلْكِ وَجَرَيْنَ بِهِم بِرِيحٍ طَيِّبَةٍ وَفَرِحُواْ بِهَا جَاءتْهَا رِيحٌ عَاصِفٌ وَجَاءهُمُ الْمَوْجُ مِن كُلِّ مَكَانٍ وَظَنُّواْ أَنَّهُمْ أُحِيطَ بِهِمْ دَعَوُاْ اللّهَ مُخْلِصِينَ لَهُ الدِّينَ لَئِنْ أَنجَيْتَنَا مِنْ هَـذِهِ لَنَكُونَنِّ مِنَ الشَّاكِرِينَ (10:22)
17:67 And when harm toucheth you upon the sea, all unto whom ye cry (for succour) fail save Him (alone), but when He bringeth you safe to land, ye turn away, for man was ever thankless. - وَإِذَا مَسَّكُمُ الْضُّرُّ فِي الْبَحْرِ ضَلَّ مَن تَدْعُونَ إِلاَّ إِيَّاهُ فَلَمَّا نَجَّاكُمْ إِلَى الْبَرِّ أَعْرَضْتُمْ وَكَانَ الإِنْسَانُ كَفُورًا (17:67)
9- Similarly if to dive in the sea with very cloudy weather you will not see your hands: it is a similitude to explain how those who live astray from God live in darkness.24:40 Or as darkness on a vast, abysmal sea. There covereth him a wave, above which is a wave, above which is a cloud. Layer upon layer of darkness. When he holdeth out his hand he scarce can see it. And he for whom Allah hath not appointed light, for him there is no light. -
أَوْ كَظُلُمَاتٍ فِي بَحْرٍ لُّجِّيٍّ يَغْشَاهُ مَوْجٌ مِّن فَوْقِهِ مَوْجٌ مِّن فَوْقِهِ سَحَابٌ ظُلُمَاتٌ بَعْضُهَا فَوْقَ بَعْضٍ إِذَا أَخْرَجَ يَدَهُ لَمْ يَكَدْ يَرَاهَا وَمَن لَّمْ يَجْعَلِ اللَّهُ لَهُ نُورًا فَمَا لَهُ مِن نُّورٍ (24:40)
10- The sea is also used to as a similitude to the words of God: if the seas was the ink to write the words of God they will be exhausted. this explain how much knowledge and wisdom God have.
18:109 Say: Though the sea became ink for the Words of my Lord, verily the sea would be used up before the Words of my Lord were exhausted, even though We brought the like thereof to help. قُل لَّوْ كَانَ الْبَحْرُ مِدَادًا لِّكَلِمَاتِ رَبِّي لَنَفِدَ الْبَحْرُ قَبْلَ أَن تَنفَدَ كَلِمَاتُ رَبِّي وَلَوْ جِئْنَا بِمِثْلِهِ مَدَدًا (18:109)
11- When we corrupt in the land and sea God punish us harshly:
30:41 Corruption doth appear on land and sea because of (the evil) which men's hands have done, that He may make them taste a part of that which they have done, in order that they may return. ظَهَرَ الْفَسَادُ فِي الْبَرِّ وَالْبَحْرِ بِمَا كَسَبَتْ أَيْدِي النَّاسِ لِيُذِيقَهُم بَعْضَ الَّذِي عَمِلُوا لَعَلَّهُمْ يَرْجِعُونَ (* http://www.nizamulislam.com/articles/english/miracles_of_quran/miracles_of_quran.asp