The usual thing in the Gulf of Mexico is for the autorotation to go fine, the helicopter floats with no damage, until the boat gets a line on it & pushes the throttles forward, whereupon the helicopter turns turtle. Max speed on the floats is probably < 5kts, closer to 1, but the boat captains are impatient with that speed, & almost always the helicopter ends up upside down, with the blades spinning, then cutting the tailboom. I saw one B206 that had just had an engine decel just after takeoff, the pilot put it in the water with absolutely no damage, and by the time it was back on the platform helideck the blades were about 3' long, it had no tailboom & no skids, & was a total wreck. The CG is very high, & the helicopter will roll in a heartbeat. I can't imagine one sinking upright, since the CG will be much closer to the head even with the fuselage filled with water, but I guess anything is possible. I certainly wouldn't want to bet my life on its sinking right-side-up.