One of the guys who did the Canadian waterbirds course where they stick a Sea King in the water deliberately, told me that they "landed" gear up and gear down and it made little difference - obviously they weren't shutting down! They did one running take-off and realised they had left the gear down once they had got airborne which I thinks demonstrates how little overall additional drag there would be if you were to ditch with forward speed.
I have spent many hours over the sea with retracts and it never concerned me if we were forced to fly with the gear locked down for some reason. I now fly over the sea in a skidded aircraft and I don't think they would make a jot of difference in a ditching situation - hope not to find out though!