Sea creatures were recently added to Animal Crossing: New Horizons in the game's Summer Update - Wave 1, and indeed many players are now busy trying to catch all 40 different species of this new type of critter. However, before some fans commit to hunting down all of the new sea creatures in ACNH they may want to know exactly what they will get for doing so, and full details on the matter can be found below.
To get right to it, upon donating the 40th and final sea creature to the museum in Animal Crossing: New Horizons, players will receive an enthusiastic "thank you" from Blathers himself. The adorable owl will then embellish further upon his gratitude, saying that that the fan's "hard work is well and truly appreciated" and indicating that the island community is "forever in [their] debt."
Indeed the reward for catching all of New Horizons' sea creatures (beyond the reward of having completed the exhibition, of course) is simply some words of gratitude, and that may feel like a bit of a letdown to some players. Certainly, something akin to a Golden Wet Suit or Snorkel, which could have allowed fans to swim faster in ACNH, would have been a wonderful prize for capturing all of these new species, and fans may be slightly surprised to see no such item offered.
That said, this is all very similar to what occurs when a player collects all of the fossils or art in New Horizons, and thus it is definitely not without precedent. Even still, some players may have been hoping that Nintendo would break tradition with sea creatures and provide an item as a reward for completing the collection. Unfortunately for these hopeful fans, the developer did not.
To close, it is worth mentioning that it is currently not possible to catch all of the sea creatures in the game naturally, and players that want to finish their museum exhibitions right now will need to be willing to time travel in ACNH. However, the lack of any major reward for finding all of the sea creatures means that fans are likely better off taking things slowly and catching these species in due time.
Animal Crossing: New Horizons is available now for Nintendo Switch.