Any machine designed for planetary combat would need to be sealed, not only against water but against the fine powdery sand present on the Moon, Mars, and probably elsewhere. Cylons and Daleks (technically cyborgs, if not organisms in mini-tanks) certainly wouldn't let liquid water compromise their effectiveness. R2 units were designed for the rigors of combat, albeit in a supporting role, so sealing off vital components improves their durability too. Not to mention, anything sealed against vacuum will also keep water out.
Also, Cylons, Dalek travel machines, and R2 units are products of an advanced technological culture, so perhaps their manufacturing processes seal off water-sensitive components, either as a byproduct or as a trivial step. Currently, waterproof electronic and mechanical devices cost extra, but it's hardly a cutting-edge technology. Then, too, modern electronics generally consist of sealed IC chips soldered onto exposed circuit boards; future manufacturing might produce completely sealed circuit cubes, optical components immune to water, or who knows what else.
"Don't get into a spaceship with a madman. Didn't anyone ever teach you that?" -- The Doctor, "Journey to the Center of the TARDIS"