@cor3zone
That's true, however, going by how they basically are scrubbed down by Tatara when he works on their blades, I think we can safely assume that when their handles are cleaned that they have their face and subsequently their teeth/mouth as well
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).
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.
@EaterOfBooks Did I missed any mentioning of iron inclusion in them? That's the thing: rust is oxidised iron, I don't think metalsmith would call every metal corrosion "rust".