How to swim in the ocean, and what you can do while swimming.
Get a Wet Suit
To be able to swim in the ocean, you need a wet suit. You can buy one from the shop, or from Nook Shopping.
Put the wet suit on and go to the beach. Press A to get into the water. Use the control stick to change direction and swim slowly. Press and hold A to swim continuously. Tap A repeatedly to swim more quickly. Press Y to dive underwater, and press Y again to come back up.
Catch Deep Sea Creatures
Look for bubbles in the water. Swim near them, then press Y to dive. Look for the shadow in the water, and go toward it while holding or tapping A. Once you touch it, you will catch it and come back up to the surface. You can donate deep sea creatures to the museum.
How to Catch Fast Sea Creatures
Some sea creatures swim very quickly and are difficult to catch. You can try making them go into a corner of the swimming area to trap them, or you can swim slowly by using only the control stick, to sneak up on the bubbles, then press Y when you are above the bubbles. You can also try pressing A just before pressing Y, to dash forward before you try to catch it.
If you catch a scallop, you will meet Pascal.