Warm seawater is very corrosive - and it's a scientific fact that doesn't depend on your worldview
That's why Boeing's Starliner spaceship potentially has it easier in the terms of re-usability - because it's designed to soft-land on land, and not splash down. SpaceX, as usual, does it a little bit differently, although it's not entirely their fault - NASA just didn't feel comfortable with fully powered landing. Why did they decide to go all the way back and splash down is beyond me; probably, being short on cash and tech, and not feeling they'd convince NASA that they can do a rocket-assisted landing on land anyway.
Russians actually did land some spacecraft on water - the BOR series of reuseable test vehicles, in the Indian ocean, back in the 70s (and I don't know the reason for that, must check and ask people in the know), but the actual vehicle (that later became the Buran) was of course designed to land on a runway.