frogman
Not to be confused with Frogman (manga).
A frogman, also called a combat diver, is a soldier trained operations involving swimming and scuba diving. They typically use a close-circuit rebreather for stealth purposes, since an open-circuit scuba generates bubbles and noise. Examples of units who employ frogmen include the U.S Navy SEALs and the Danish Frogman Corps. AN-94's "The Diving Bell and the Doll" costume from Girls' Frontline is clearly meant to be a frogman.
Only use this tag if a character is wearing equipment that is clearly for military use (e.g. Danbooru post #3151793) or it is known that they are an active soldier (e.g. AN-94)