A quick summary of comments:
1) Wessex types were notorious for losing float bags on anything other than a flat entry at low speed. We lost a Wx5 off Norway at night and although the bags inflated as advertised, the force of the impact (Inadvertant Flight into Water at high speed at night) tore one off and the other soon deflated - it sank like a brick, both crew lost, sadly. If the ditching it controlled and level (in good/moderate sea conditions) the Wx would sit fat, dumb and happy as long as you didn't do anything daft like engage the rotor brake, which oftens turns the higher c.g. aircraft turtle - not recommended practice - let the blades coast to a standstill or hit the water - it's always safer and I have even know crew members who jumped out before a controlled ditching (UH1) to then get back in again afterwards once the blades started to trim their hair some on the surface! There were several Wx recovered successfully/undamaged after controlled ditchings.
2) BELLs tend to ditch nicely if well controlled on entry and as long as the floats aren't manually deployed in flight beforehand. This is because many of the flot bags (skid mounted) have lengths of plastic inside to stop the bags from adhering to themselves in the heat etc. The AB205/UH1 thus would dump 6 to 8 feet of plastic sheeting into your tail rotor during final autorotation (unless your timing was immaculate!). The 412 is better and has been recovered successfully several times after controlled ditchings. The only failures being when rescue craft have subsequently puntured the floats by 'coming alongside' under the stationary rotor disc thinking that the floats are bloody fenders! Best to get everyone over the side into as many liferafts as you have as quickly as possible, and then be recovered from the liferafts - saves the aircraft in many cases and stops people sitting in the ditched machine waiting for the inevitable to happen!
3) The S76 floats well and has great float gear - as demonstrated by many. It will also block the pilot's door from opening however and is most effective at stopping the aircraft if inflated (accidentally) during ground taxy! More difficult to puncture than the Bell/Wx, the spread and size of these puppies will normally keep everyone's feet dry! (As opposed to the wasp/scout with it's flotation level 1 feet below the surface!).
4) Blackhawk. This Beast is a 22,000lb lead sinker - so unless you have flot gear fitted (which few have), don't expect it to keep you on the surface in the event of a ditching. The ESSS stubs and tanks would likely detach anyway (they're only only Glass-fibre) and would probably just ensure a nice level glide down to the sea-bed! I would recommend underwater escape bottles as a standard (as in the USN) for any pilots flying the Beast offshore!
So - recommendations are, in a controlled ditching, let the floats work automatically if they do so, or operate at the last minute otherwise, don't use the rotor brake at all, get into the liferafts IMMEDIATELY and remain attached to the aircraft (if still afloat, that is!) until rescue arrives. Then keep the rescue tug/boat aways from the flotation gear - so that you get a bonus for saving the aircraft for salvage! ALERT DISASTER CONTROL run a very good HUET course in Singapore and might help with any queries.
