and then you can't re-use the capsule because sea water is so corrosive
It's not that bad. I handle the materials of aerospace hardware that work in a salt spray-and-sulfur environment (from burned high-sulfur hydrocarbons), and it just takes some care: watch out for dissimilar metals in contact; use appropriate surface modifications (anodizing, chem film, passivation, etc.); use appropriate paints (primer, top coat); conformal coatings for electronics; seal up well if you can; avoid moisture entrapment spots; avoid corrosion-prone alloys (2000-series aluminum; anything magnesium; carbon steel); etc. Verification is usually by months in a chamber per ASTM B117 or a few days per ASTM G85.
Target life of the hardware is 20 years in the that environment, and I generally use a lot of the materials SpaceX would.
Saltwater immersion is a complication, don't get me wrong. My life would be a lot easier if customers kept their hardware in cold, 0% humidity deserts. :) But it's an addressable complication.