Again... Why, Why would you take a delicate steel blade near water ?

Claverhouse said:

Again... Why, Why would you take a delicate steel blade near water ?

At least with Servants, the blade is a mass of solidified magical energy and manifested legend rather than delicate steel. Even if the historically accurate blade would rust or otherwise become damaged by exposure to the elements, a Servant's weapon (especially a Noble Phantasm like Ushiwakamaru's sword Usumidori we see here) can be abused in ways that would drive historians, blacksmiths, and martial artists completely mad and have no effect on the weapon. Slicing through stone, steel, deflecting/cutting bullets, and fighting in extreme conditions or underwater doesn't impede or harm the weapon much at all.

Really, the only things that can outright significantly damage such weapons are other magically imbued weapons or entities of similar potency. Mere water will only damage the weapon if the legend behind it is very clear that it was destroyed by water.

Seeing as how Summer Ushiwakamaru is still a ridiculously devoted Servant even on vacation, she always has her weapon handy just in case even while having fun at the beach.