It really depends on the atv and the tires. For example, my atv is a Honda Foreman 500, with 26" 589's
http://www.amazon.com/ITP-Utility-Sn.../dp/B001CD8MLI on Aluminum rims, rig weighs about 650lbs with the tires, only the rear rack on my rig was at the surface, it had a nose angled down attitude while floating. With stock tires it would have sunk like a stone, even at the 200lb lighter weight. I would have taken a picture of it, but my cell and wallet were in the fender box of the atv (came out dry as a bone, rear box was completely full of water).
The old three wheelers with the 25 inch tires would routinely float, straight up and down.
I guess it depends on what you are looking for as far as floating, riding safely accross the top of the water like one of those three wheeled beach bikes, NO. Staying somewhat near the surface, possible, but there really is only one way to find out, put your trunks on.