Do this while also wearing a towel around your waist (if guy) or chest (if girl). If the life guard says no, cover yourself with your arms (as if to keep the towel from falling) and walk back to change room pretending to be embarrassed.
Use my advice above. If they say yes you can still save the joke and quickly remove the towel to make it look like you're flashing them, only to show you were, in fact, wearing a bathing suit the whole time.
For extra fun at the pool, set a stray bikini top loose in the water and watch as every guy around proceeds to give himself whiplash. Whenever someone gets around to fishing them out, go "Oh THERE it is!" and rush over. While that may not sound insane, doing so as a hairy-ass dude will send half that pool to therapy for a month, mark my words. Wear the top back out and while looking completely nonchalant for Chuck Norris points. Remember to color coordinate!