Nope, paintball guns! Red paint. Getting wet doesn't send a message. Ever get wacked with one of those without thick clothing and man that will leave a welp!
And why should not someone who uses a paintball gun against another without consent be prosecuted for such? Even a water pistol would be a somewhat shaky ground legally (if a local prosecutor is anti-gun, he'd probably cooperate with any 'lie in' participants who wanted to press charges; I doubt he could score a conviction, but he could still impose thousands of dollars in legal expenses).
I'd suggest a toy "laser pistol" might be better. No real way to get prosecuted for that I would think. Though what might be better still if there were two 'lie ins' near to each other would be to have the 'attacker' attack both, but have an "armed" person nearby the second one.