The weird wildlife in the Blue Hole are mostly the result of ancient Sea People experiments.
While this is strongly implied, I don't think it's ever actually confirmed in-text beyond the final boss, so here is a WMG. This explains the hyperaggressive sharks (and other wildlife), the electric jelly fish, and the narwhals that lay shark eggs, although the latter was patched out where the eggs were replaced with baby narwhals.Dasche isn't dead and will become a major villain in a possible sequel/DLC (if he isn't the Giant Gadon).
He's experimented a lot on the Sacred Tree fruit which has a longevity/possible immortality effect, as shown by his experiments the Gadon and Yawie still being alive after 800 years. What's not to say that Dasche used it to extend his own life, except in a more refined process that doesn't turn him into a mindless Gadon? It's also never mentioned if Dasche actually died when Tenzhin retells Dave about the origin of the Gadon.The "narwhals" aren't actually narwhals.
They're narwhal-shark hybrids that were the result of some of Dasche's experiments.