Snorkeling is a new feature that was added in The Sims 4: Island Living expansion pack. You do need to own and have installed this expansion pack before you will be able to snorkel. Once that's done, here how you snorkel in The Sims 4: Island Living.
Before you can snorkel, head to the new area called Sulani. You won't be able to snorkel on the regular areas from the other expansion packs or the base game. When you are there, head to an open body of water. Look for a buoy in the water. You need to interact with this and select the "Snorkel" option that pop ups. Not all buoys will allow you to snorkel, since some are reserved for diving only. However, most of the ones close to the shore allows for snorkeling.
When you select that option, your sim will automatically begin to snorkel. It's a non-interactive activity, so you only get to watch while your sim perform the snorkeling.
And that's all there is to snorkeling in The Sims 4: Island Living. Enjoy your underwater adventures!