Just guessing: In space the pressure comes from inside, in water from outside; it is easier to protect in one direction than in both. In space the pressure difference is at most one atmosphere, in water it depends on how deep you submerge. The salt water also attacks the metal chemically.
Just guessing: In space the pressure comes from inside, in water from outside; it is easier to protect in one direction than in both. In space the pressure difference is at most one atmosphere, in water it depends on how deep you submerge. The salt water also attacks the metal chemically.