Other forms: submersibles
Use the word submersible to describe something that can be used under water, like your fancy new submersible video camera.
If you can use your watch while you're scuba diving, it's submersible, and so is all of your diving gear. Anything that functions in the water can be described this way, including a submarine, which is sometimes actually called a submersible, as are other machines designed to work under water. The word comes from the Latin submergere, "to plunge under or sink," from the root words sub, "under," and mergere, "to plunge or immerse."