Okay kiddo, so a combat diver is someone who learns how to do special swimming and diving techniques in order to help with military missions. They wear special gear like flippers, a mask, and a diving suit to help them breathe underwater.
They might have to do things like sneak up on an enemy ship or plant explosives on something that is underwater, and they have to be really good at staying hidden and not making any noise.
Being a combat diver is really hard work, and takes a lot of training and practice. But they are very important to helping the military do their job safely and effectively!