As far as the UK is concerned, the rules for the London Heli-lanes are a good indicator of how the CAA view this. Some of the routes involve flying over the River Thames. If the engine stops, the designated forced landing area is between the high and low water marks. If you choose to fly a single engined helicopter when the tide's in, it's your choice and no-one else's responsibility. But you must ditch! Floats are a good idea...