As you explore your island, and other islands, in Animal Crossing: New Horizons, you will discover that your greatest enemy is water. You cannot cross it without the right tools, and in many ways, it will be a constant nemesis.
Despite the ever-present water in the game, you were unable to swim or dive in Animal Crossing: New Horizons when the game originally launched. The Summer Update has added the ability to swim and dive in the game. If you go to the terminal in the Nook Shop, you will be able to purchase a swimming suit that allows you to enter the ocean and go for a swim. Equip the swimsuit from your tools, then face the sea and press to enter the water, then Y to dive under the water.
You will need to use some other means to get around water on your island. To cross rivers, you will need to get a Vaulting Pole. To get this useful tool, you will need to have unlocked the first stage of the Museum, which requires you to give 5 different species of Fish or Bugs combined to Tom Nook to make Blathers come to your town.
Visit the newly opened Museum and speak with Blathers. He will then offer you two new DIY Recipes which include the Shovel and the Vaulting Pole. When you b build the Vaulting Pole, you will be able to use it to cross rivers, and visit parts of the island that had been cut off until now.
Unfortunately, the sea will remain as imposing as it always is, and you won’t be able to swim. You should regularly visit the beach to find shells, bugs, butterflies, and stranger things.
If you want to visit other islands, you will need to visit the airport, as that is the only way to cross the sea. Dodo Airlines will be able to ferry you off to mystery islands, or your friend’s homes for a visit.