That happens because water gets inside the horn on the diaphram, changes its accoustic characteristics. I think people with those unprotected hella/spyder/etc horns deal with all the time, but theirs probably also dry on their own relatively quickly.

You could either remove your own horn and drain it/let it dry out or try taking it to the dealership and have them do it (or maybe they'll just give you a new one, who knows.)
Keep in mind, it is easy to remove your horn (easier than dealing with a dealership). Also, its a good opportunity to install an additional low tone horn to make your car sound less pathetic (assuming gen2s suffer from the same problem as gen1s)