As the name suggests, marine creatures are creatures that can be found on the seabed by diving. They have their own Critterpedia page, although most of them are fish. To capture marine life in Animal Crossing: New Horizons, you need to purchase a wetsuit. You can buy one using ACNH Buy Bells.
If you are not interested in keeping your catch, you can sell marine life for cash. There are even some creatures whose prices are extremely high, and players can use cash to buy other useful items.
How to catch marine life
After you have your wetsuit, click the "A" near the ocean, equip it and dive into the water. You can jump off the rocks, or more easily, wading peacefully from the beach. When swimming, you will eventually encounter bubbles. Swim over the bubbles, then press "Y" to dive down, you will encounter marine life. If the shadow starts to move away, you can continue to swim underwater by tapping "A", and once you exceed the shadow, you can capture elusive creatures by pressing "A".
The type of marine life captured in "Animal Crossing: New Horizons" is determined based on the specific month, season, and time of day you currently experience in the game. If you still don't get the ideal bio, you can trade with Buy Nook miles Ticket.