Crab,
I was away for a week, thatīs why I can only now answer.
Prolonged in this case means within autorotative range.
If you fly along the coast, Iīd not be further out than a hundred meters or so, to make sure that at least someone sees you going down. Swimming 100 m in the Baltic during winter in your normal clothes is a no go, but again, flying in itself is dangerous...
Having done my initial helicopter training @ DHFS, we were drilled not to fly over lakes, wooded areas and the like.
Annoying at times, but saving lives and probably lessons learnt the hard way.
Tom