In the abyss, there's a small chamber with a couple 'weird fish' who stare unceaseingly at glowing orbs. Is there something to do here or should I just leave them alone?
You can tap/click/whatever them and see what happens. It's nothing important, just a little easter egg.
Also, is there anything to be done with the whale directly outside the fish room?
Yes, it's optional but you'll need it for 100% completion and an extra.
Hint: Somewhere in open waters is a stone tableau that has a 'whale' song inscribed...