Sea creatures, like fish, have a shadow that varies in size. Additionally, bubbles can be seen above a sea creature when swimming, and they may move slowly, quickly, erratically, or be completely stationary.
There are a total of 30 sea creatures in Animal Crossing: New Leaf, and 13 more were added in Animal Crossing: New Horizons, while three (the Ear Shell, Clam, and Horsehair Crab) were removed. While initially absent from Animal Crossing: New Horizons, sea creatures, along with swimming and Wet Suits, were added in the 1.3.0 Free Summer Update in July 2020.