Braking is diminished in the rain, regardless of whether you are skid/skipping or hand braking. Nonetheless, a hand brake is still more effective overall. If your hand brake is less effective at stopping you than your legs, then it needs adjustment or new shoes or something....
Having said that, I always will avoid using a handbrake in the rain because it accelerates rim and brake shoe wear big time and makes lots of black rim gunk. The gunk spreads to your tire sidewalls, frame, fork, and everywhere else.
I have always despised black rim gunk and I will carry that hatred to my grave...