I'll call your Lemme ask you something real quick: who wanders around on a fishless sea? and They are vast seas... None can swim across them... They yield no fish to catch... and raise several quotes that suggest there are fish in the sea.Those treasure hunter guys say that there are no fish in the sea. If that's true, that might be your reason. The biggest reason the Zora would have to keep swimming the sea is to fish. And now all there were in the oceans were monsters. No fish. If fishing was no longer possible, they might not go into the water as much. And eventually became the Rito.
[Outset Island] is the most delightful of fishing villages, innocent and rich with nature's bounty...
I can only assume that "nature's bounty" here refers to fish.
[Orca] soon returned to Outset and became a fisherman.
Why become a fisherman if there are no fish?
And you will NEVER guess what happened! This fishy-looking thing popped up on the surface, but it was no ordinary fish! It had this weird, human-looking smile on its face, and it just gobbled up the bait!
"...it was no ordinary fish!" implies that there are ordinary fish. Also the fishmen are frequently referred to as fish.
You'd better remember this! Even if you see a shadow underwater that you THINK looks like a fish, whatever you do, don't throw bait out for it, OK? Because if you do, one of the dreaded fish-men might appear and pull you underwater with him!
Again, there are other fish besides the fishmen. "might appear"
Seahat [...] These tremendous flying fish can be found only on the Great Sea.
Monsters are fish too!
I dismiss Ganondorf's quote as a metaphor (he's talking about a dream), though I can't explain why the treasure hunters think there aren't any fish. In the face of all this evidence, I think they're wrong.