A sealing device for a body suit and a sealing method utilize a reactive seal that incorporates a swelling polymer activated upon contact with a fluid medium, such as water. The reactive seal can be embodied in a neck seal, wrist seal, or ankle seal of the body suit, where the reactive seal is designed to be loose and comfortable to wear, only exerting sealing pressure when needed. The swelling polymer can be a superabsorbent hydrogel such as a blend of polyacrylate polymer with poly-anionic beads (PAB). In one application, a 50/50 blend of polyacrylate and PAB is used, such that the reactive seal can autonomously tighten within about 10 to 15 seconds. The device and method can be used in various other applications to form a seal for preventing the passage of fluid from one volume to another.