Protect your feet and keep them warm

In the same way a wetsuit will keep your body warm, boots will keep your feet warm. They also protect your feet when you're walking, for instance, on a beach before or after a shoredive.
The boots mustn't be too small or else you'll get cramps while scuba diving. The fin pockets also needs to be big enough to stick your foot in when wearing the boots.
These boots are made of neoprene and usually have a reinforced sole made of rubber.
Even boots come in a variety of thicknesses, but most common are 5mm boots.