I think that minecraft should have another sea creature.Fish could be be seen in water. A good idea for a sea creature is a puffer fish that puffs up when you go near it. Another good idea for sea creatures are sea dragons that attack the players and their boats, but are not too difficult to slay .
This idea is from the Mo' Creatures mod but maybe you could tame tortoise with melons and ride them under water (with a aqua infinity helm of course). Maybe tortoise could swim in the murky water of the swamps.

Crafting ideas:
~PufferFish would drop PufferFish Thorns (the spiked points) and then they could be crafted with sticks to make Thorned Dagger ( One hit = 3 hearts of damge to an unarmored player)
~Sea Dragons could drop AquaScales that could be made into Aquatic Armor, which is one have of a armor bar better than iron.