Currently, the water is boring in Minecraft. It's dark, bleak, only has squids for mobs and the only danger in drowning. I think it's time to change that. New mobs, new environment features and new structures underwater. I believe that these features would add new dangers to the game, and add the beauty to the water, as it deserves.
Mobs
New mobs could be added into the game, such as:
Fish
Fish could be added into the game fully, instead of having merely the fish item. Different fish could spawn in different biomes, like have hostile piranhas in jungle rivers. These could add a bit of life to the water and an extra precaution to be weary of when entering the waters. When caught with a fishing rod, the same fish item we get now would fly into your inventory, waiting to be cooked/fed to ocelots.
Jellyfish
Jellyfish would be hostile creatures, spawning in large numbers in the deep ocean. Swimming into them would deal damage to the player. They would no seek out the player. They would not chase the player. Jellyfish would simply lie in the water, waiting for a player to hit them.
Dolphins and Sharks
Dolphins could act as wolves do, tameable and neutral. When provoked, they attack in numbers and with ferocity. Sharks would be solitary mobs, that are aggressive towards the player, and, if in the vicinity, begin to swim towards the scene of a water mob death. Both mobs would drop meat to be eaten.
Turtles and Alligators
Turtles would be passive creatures, and would spawn near the coast and often in swamps. Alligators would be hostile creatures, and spawn in the same area of the turtles. Turtles would drop their shells upon death, and alligators would drop leather. The turtle shells would act the same way as mob heads, being able to be placed as a block or worn on the player's head.
Environment
Naturally spawning structures could spawn in the water.
Currents
Naturally flowing currents that flow a certain direction, and become stronger according to the biome, ie flowing a bit calmly in forest biomes, and flowing faster in jungles.
Whirlpools
Naturally spawning whirlpools that spawn in the open ocean. They would vary in size, be funnel shaped, and have a chance to flow into an underwater dungeon.
Dungeons
Dungeons would be found underwater, and have a chance to spawn whirlpools above it, acting as an entrance into the dungeon. These dunegons would have various spawners in them, and hold chests with treasure in them.
Coral
Coral would spawn in vast numbers on the ocean floor and, as the mod by Nandonalt has, they could provide light underwater. These coral reefs would add light and beauty to the otherwise dark and bleak oceans.
I like the idea of fish or turtles, but your descriptions are fairly vague. What do they drop?
I also approve of the idea of whirlpools - they could create another threat besides drowning - however, I do not like how it always enters into the dungeon. It would be too obvious in my opinion; how would you get out of the dungeon . . .?
Mobs
New mobs could be added into the game, such as:
Fish
Fish could be added into the game fully, instead of having merely the fish item. Different fish could spawn in different biomes, like have hostile piranhas in jungle rivers. These could add a bit of life to the water and an extra precaution to be weary of when entering the waters. When caught with a fishing rod, the same fish item we get now would fly into your inventory, waiting to be cooked/fed to ocelots.
Jellyfish
Jellyfish would be hostile creatures, spawning in large numbers in the deep ocean. Swimming into them would deal damage to the player. They would no seek out the player. They would not chase the player. Jellyfish would simply lie in the water, waiting for a player to hit them.
Dolphins and Sharks
Dolphins could act as wolves do, tameable and neutral. When provoked, they attack in numbers and with ferocity. Sharks would be solitary mobs, that are aggressive towards the player, and, if in the vicinity, begin to swim towards the scene of a water mob death. Both mobs would drop meat to be eaten.
Turtles and Alligators
Turtles would be passive creatures, and would spawn near the coast and often in swamps. Alligators would be hostile creatures, and spawn in the same area of the turtles. Turtles would drop their shells upon death, and alligators would drop leather. The turtle shells would act the same way as mob heads, being able to be placed as a block or worn on the player's head.
Environment
Naturally spawning structures could spawn in the water.
Currents
Naturally flowing currents that flow a certain direction, and become stronger according to the biome, ie flowing a bit calmly in forest biomes, and flowing faster in jungles.
Whirlpools
Naturally spawning whirlpools that spawn in the open ocean. They would vary in size, be funnel shaped, and have a chance to flow into an underwater dungeon.
Dungeons
Dungeons would be found underwater, and have a chance to spawn whirlpools above it, acting as an entrance into the dungeon. These dunegons would have various spawners in them, and hold chests with treasure in them.
Coral
Coral would spawn in vast numbers on the ocean floor and, as the mod by Nandonalt has, they could provide light underwater. These coral reefs would add light and beauty to the otherwise dark and bleak oceans.
Otters are fantastic
Please read these two threads:
http://www.minecraftforum.net/forums/minecraft-discussion/suggestions/2572194-please-read-this-before-making-a-suggestion-v2-0
http://www.minecraftforum.net/forums/minecraft-discussion/suggestions/44180-for-the-critics-ftc
I also approve of the idea of whirlpools - they could create another threat besides drowning - however, I do not like how it always enters into the dungeon. It would be too obvious in my opinion; how would you get out of the dungeon . . .?
Otters are fantastic
Much better, love the turtle as helmet ideas, dolphin, and sharks.