There is a spell for everything -- well, mostly everything. We're told to not rely on magic exclusively. There could always be a day where you don't have it available to you.

[Tom raises his wand through the water to point it at Morgana.]

Impervious.

[Her clothes are now water-proof, just as Tom's are, and he lowers his arm with a tin smile.]

Though I've yet to come to that point myself.