Scuba diving and snorkeling boots are made of neoprene, which contains tiny nitrogen bubbles inside the material. These bubbles form an insulating barrier between your body and the water, proving excellent thermal protection.  A thin layer of water will enter between your boot and your foot at the beginning of your dive, which is quickly warmed to your body temperature.  In order to keep you warm, boots must fit snugly, so that this warm water layer is not constantly flushed out by colder water.  Wetsuit legs should be worn over the top of the boot to reduce water flow into the boot while diving.

As you dive deeper, your boots will compress due to the increasing water pressure, which makes your boots lose some of their insulating capability.  Therefore, you should consider buying boots that are designed to be warm at the deepest part of your dive.