Abstract: A seal is provided for an offshore fluid swivel to seal a gap passage (34) leading from a transfer chamber (20) to the environment, which uses pressured fluid to apply a variable pressure that pushes the seal tightly against a sealed surface (52) on one side of the gap passage. In one arrangement, a ring-shaped seal device includes a seal body (82) of largely U-shaped cross-section, with a base (84) and a pair of legs (86, 88) extending from opposite sides of the base, with the base and at least one leg sealing against walls of the cavity that holds the seal. A hose (90) lies between the two legs and against the base of the seal body, and pressured fluid of greater than chamber fluid pressure is applied to the hose to expand it and thereby push apart the legs of the seal body.