In order to swim and dive to the ocean floor in Animal Crossing: New Horizons, you’re going to need a wet suit. Before you begin, make sure your game is updated to the latest version. Diving was added to ACNH with the 1.3.0 patch.
Once successfully updated, you’ll get the following message from Nook Inc. in you inbox:
With that, you can head to Nook’s Cranny. Despite the wet suit being a piece of clothing, you’ll actually need to buy it from Timmy and Tommy. You will not find it at Able Sisters. This is likely because you have to unlock Able Sisters’ tailor shop, which would prevent players from accessing swimming and diving until they unlocked the shop.
Once purchased, you can wear the wet suit and enter the ocean for a swim!