That happened at least three times that I remember. They tended to flip over once the rotor was shut down, but they would generally float for awhile. There was also a "horse collar" floatation device that could be attached by swimmers which would keep it floating quite a bit longer.
Our squadron got one of those (ditched, then reworked at NARF). It had been in the water (upside down) for about 6 hours. Unfortunately, salt water had gotten into one of the turnbuckles on the tail rotor cable. Replacing those was not part of the checklist for this level of rework ( I found that out when doing the JAG on the mishap); it corroded from the inside, and broke just as the helo was lifting into a hover. Luckily the crew responded quickly and correctly, (shut the engines off ASAP and hope/pray!) and no one was hurt, although the helo was damaged pretty badly.