The Blessings and Signs of Allah
Allah says, pointing out His mighty power in creating different things, how He has created the two seas (kinds of water). The fresh, palatable sea (kind of water) refers to the rivers which flow among people, rivers great and small according to people's needs in all regions and areas and lands. This water is sweet and palatable for whoever wants to drink it.
وَهَـذَا مِلْحٌ أُجَاجٌ
and that is salty and bitter. means, unpalatable. This is the ocean in which the big ships sail, and is salty and undrinkable. Allah says:
وَهَـذَا مِلْحٌ أُجَاجٌ
(and that is salty and bittery.) Then Allah says:
وَمِن كُلٍّ تَأْكُلُونَ لَحْماً طَرِيّاً
(And from them both you eat fresh tender meat,) meaning, fish.
وَتَسْتَخْرِجُونَ حِلْيَةً تَلْبَسُونَهَا
(and derive the ornaments that you wear.) This is like the Ayah:
يَخْرُجُ مِنْهُمَا الُّلؤْلُؤُ وَالمَرْجَانُ فَبِأَىِّ ءَالاءِ رَبِّكُمَا تُكَذِّبَانِ
(Out of them both come out pearl and coral. Then which of the blessings of your Lord will you both (Jinn and men) deny) (55:22-23).
وَتَرَى الْفُلْكَ فِيهِ مَوَاخِرَ
And you see the ships cleaving, means, they travel through it, plowing through the water with their beak-shaped bows. Muja0hid said, The wind drives the ships, and the wind cannot drive any ships except the big ones.
وَلِتَبْتَغُواْ مِن فَضْلِهِ
that you may seek of His bounty, means, through your journeys to engage in trade from one land to another.
وَلَعَلَّكُمْ تَشْكُرُونَ
and that you may give thanks. means, that you may give thanks to your Lord for subjugating this mighty creation -- the sea -- to you, so that you can travel through it as you wish and go wherever you want, and nothing stops you؛ His power has subjugated for you whatever is in the heavens and on earth, and all of this is by His grace and mercy.
Tafsir Ibn Kathir (abridged, English).