Why the fantastic fish?

For me, & I love the choice, it seems to set the film in a fantasy reality of sorts.
Even the underwater lab is quite fantastical, & their quirky little submarine, a sense of the fantastical dots the film nicely.
Because it's all consistent, it never ever distracts.

...my essential 50 http://www.imdb.com/list/ls056413299/

It's in keeping with his visual style.

I think it's because he wants the underwater sequences to play like a child's fantasy of being an underwater explorer, not like reality. It's a fantastical film, deliberately unrealistic, and he wanted something visually spectacular, not the real underwater world* of darkness below 10', murky water, algae, and the most interesting species being hard to find and film. Take it from someone who's done a fair amount of SCUBA!


Plus, when he decided that animation was going to look better on screen than reality, he probably decided it was easier to just let his animators make stuff up than research which species could be found where. Because bitches like me would know if they got it wrong, and complain...

Just part of the whimsical tone set for the movie.

Just cuz he could.

It's a movie about a severely depressed man but the one thing that still makes him happy is the sea.

I think it was meant to make you feel the childlike wonder he still feels for it.