Sure, you can dive without flippers- but it just means that your legs are going to get tired a lot faster. Flippers serve to take a lot of pressure and strain off of all of the muscles in your legs and places the work on the fin of the flipper. If you don’t have a pair equipped, it means that your legs are going to be doing all of the work. If your legs get overly tired, you are going to want to swim back up to the surface and call it a day faster than if you had a pair equipped.