V-Brake pads for wet conditions
I've had good luck (with cantilever brakes) with Velo Orange's Squeal-Free Brake pads, but many people say that the Salmon compound brake pads from Kool Stop are the best choice for wet conditions.  The dual-compound Kool Stops are a good idea, since supposedly they work well in all conditions...

My LBS stocks brands, but Kool Stop is a much more usual brand of brake pad to carry.

In general: any brake pads that say they're made for wet conditions.