As to exact Studies Im no help.I do think by using the internet you may find some information that will be of use. I suggest downloading a free Search Tool that utilizes many Search Engines at once.
www.copernic.com
Site I would look to may be the Navy or Coast Guard. They live on the water and that is a big concern of theirs. Try
www.navy.mil or
www.uscg.mil
Having a few hundred hours overwater its also been a concern of mine albeit I flew different helos. In conversations items that came up were will they rip off in Autorotation/ Hitting the water etc. Experience of a room mate on one day out of 365 where he had calm water and close to shore. He had an engine failure in a B206 and put it down perfectly in the water off St Croix. They towed it to land.
Good Luck and if your in Angola, the water is probably prefered over walking through the Minefields.......