Damn, those two guys got really close to being meta there with "it's our duty to hit on them!" (because we're generic anime beach bum ikemen who exist only to hit on the protag's girls).
@kwendy
Steel still rusts, and even stainless steel (which is the most rust-resistant steel, but also a poor material for making usable swords out of) has corrosion issues with seawater.
Even if all three of them are entirely made of magical supermaterials, they're still metal, and seawater is just about the most corrosive environment for metal found naturally. It's also been confirmed via the maintenance scenes that whatever the girls are made of, it
can corrode or rust, and needs to be maintained properly. (Fun fact - this is why metal ships have a coating over the metal to prevent corrosion.)
At the end of the day, it's an excuse to do a suntan oil scene, but it does make some sense.