On this page of our FAQ to Animal Crossing New Horizons we explain what you have to do in order to swim in the sea.
Swimming in Animal Crossing New Horizons is extremely easy. Therefore, we'll explain what you have to do to swim through the waters around your island.
At the beginning you have to get a wet suit. You can buy it for 3,000 bells at Nook's Cranny (you can find it in the case where there are tools or seeds) or for 800 Nook Miles at Nook Shopping, that is, visit an ATM. Once you've equipped the wet suit, go to the sea. Then press the "A" button on the controller and your character will enter or jump into the water. And that's it.
Note - when you are entering the water, you cannot have any tool equipped in your hand. Otherwise you'll use the item.
Is it possible to swim in a river or lake?
No, you can only jump into the sea.
How to swim in the sea?
In the water, you can move slowly with the left knob of the gaming controller. To speed up, which means to use the front crawl swimming style, tap the "A" button from time to time. And that's basically it - the last ability that can be used is diving, simply use the "Y" button.
Can I drown in the water?
No, you can't drown in Animal Crossing. If you dive for too long, your character will simply emerge from the water at some point.