The land is filled with interesting blocks and mobs, and now, so is the water. Between the two, is null but dirt. I think there should be more in the swamp.
Sugarcane should grow not only near water, but also in water at swamps. Also, seagrass won't grow in swamps if it currently does, and instead tall grass will grow in 1m deep water.
There would be flowers that spawn in water, near to lilypad clusters.
Mud is a block that spawns in and around water. It slows down most mobs.
Frogs and toads will spawn in mud. They will also spawn as tadpoles. They jump about, rather than walk. Frogs need water and air, whereas toads only need air. They both eat flies by using their tongue to grab them. They also swim about in swamps.
Salamanders are more or less frogs that walk and don't jump.
Caecillians are worm-like mobs that crawl through mud blocks.
Mudskippers are fish-like mobs that walk on mud and swim in water, and need water but not air.
Flies are tiny mobs that fly around. They can be hurt by snowballs, but are too small to be easily hit.
Crocodiles are rare, large, hostile mobs. They swim about, but do not need water, just air. They are about 1m tall, and 4m long. They can gallop, walk, or crawl on land.
Ducks are flying mobs. They swim on water, and can walk on land. They drop raw duck.
Giant toads are 2m tall toads. They are neutral, and will become hostile if attacked. They will use their tongue to eat ducks. They can be tamed with raw duck or fish. Once tamed, they can be given a chest or a saddle. If saddled, they can be ridden on land or water. They attack by biting and leaping at the victim.
Also oceans are deep. They're three-dimensional biomes with lots of space for mobs to do mob things.
Swamps can be huge, and mobs can be small. The only problems would be giant toads and crocodiles, both could be fixed with rarity. And if Mojang decides not to make the tiny mobs unviewably intricate, they could make all the mobs with 4 models and 6 AIs.