Overall a nice conept with potential, but I still have a lot of questions though.
1) First the battle stadium. If the circles are platforms and the square in the middle is the water that Walter swims in, then what is the area outside the square where the platforms are sticking out off?
2) How do you get from the water in the middle to the platforms outside? Or can you climb up the front of the platform from whatever that area is?
3) You hit it with the boomerang to do damage?

The boomerang is mostly used to stun enemies, unless they are really weak. Does this guy have an angler or something that is weak and the boomerang can hit. And at does the bit more powerful hookshot than.
4) When Walter does the body slam and he lands on a platform, where there is no water, how does he get back in? Does he have legs?
5) The puddles of water from the spiracle rain, are they relevant to the fight or just a graphical nicety? Just a graphic is fine with me though.
6) How does ramming the platform Link is on hurt Link? Could you provide some more details.
7) When hookshotting Walter doesn't Link end up getting bump damage?

With a name like Walter, was he once the pet guppie of a zora or something

Give a bit backstory.
The description is a bit minimal. It just says a large blue whale, which seems like you want to copy Pokemon's Waillord again. You might want to work on his appearance, like distinguishing features.
I look forward to your answers and my suggestion is to also add those to the OP if you want it accepted
