One of the challenges in Animal Crossing: New Horizons AC NH requires you to collect sea creatures, added in the 1.3.0 update. There are 40 sea creatures waiting for you to find and catch. Since the sea creatures are active during certain months and times, it will take you a long time to complete this quest.
To find sea creatures in
Hold the A button to swim. If you press the A button repeatedly, you can increase the tempo a bit. Look for bubbles in the water that indicate sea creatures. When you are over a bubble, you must press the X button to dive. Underwater, touch the visible shadow to catch the sea creature.
Sea creatures have different sizes and also move underwater. Some rare animals are too fast that you can hardly catch them. Try to corner these sea creatures or cut them off. In addition to sea creatures, you can also find valuable pearls.