prop_physics_multiplayer and try it out..? there are several on 2fort i smash and a can you can shoot off teh red balcony. So they must have a physics version. I doubt the tyres will though, unless you use a hl2 version.
I can't remember if they still have a bouyancy option, but that'll (obviously) be the one you wanna put a figure in to allow it to float.