There is a difference between floating and floating upright. Without floats, the helicopter will roll over almost immediately, because the CG is so high. However, it will float at least partially for some time, depending on several factors. The amount of fuel in the tank, and the integrity of the tank, is one factor. They may float for days if the seas are relatively calm and the tank is intact and the fuel cap seals well. But you may as well face the fact that you'll likely have to exit with the aircraft upside down & the cockpit underwater. It's certainly doable, and HUET training should be a big help with this.