Sea creatures are animals you can find on the seafloor in Animal Crossing: New Horizons. You need to purchase a swimsuit (wetsuit) from Nook’s Cranny, or one from the Nook Miles Terminal at your city hall, to be able to dive in the game. After that, you can wander around the ocean, searching for bubbles to indicate there’s marine life you can pick up to give to Blathers to add to his museum collection or sell at Nook’s Cranny for some bells. Some players are curious to see if C.J., the beaver who purchases fish at 150 percent their value during fishing tournaments if he takes sea creatures too. He does not.
He does not accept any of the sea creatures you find by diving into the ocean. He will take any of the fish whenever he visits or anything you have during a fishing tournament, but the sea creatures are off the table. There’s no reason to save a massive pile of them for some bell profits or do any of these activities when visiting your island. You want to focus on acquiring fish purely.
You can feel free to sell your creatures to Nook’s Cranny for a pretty penny, but we don’t know if Timmy or Tommy will want to purchase them for an additional profit. Alternatively, you could always try to find a way to model the sea creatures on your island. It never hurts to have a vampire squid hanging out in a tank nearby, especially at night, with his glowing eyes staring back at your residents.