I find sex in the shower very painful without a lube of some kind, my own lube is usually rather watery and not very thick to begin with, water makes it much worse.

But in all honesty... sex in the shower when I am on my period is wonderful.

I have found no lube that works in water, and the very sad thing is that being in a water environment is probably one of my single biggest, most immediate, and strongest turn-ons.

*sigh*