Bing
Yes and yes, and I've seen plenty of accident reports where the doors were jammed in position post impact. The jettison isn't a magic cure all so if I had the option I'd want to jettison the door before hitting the water.
There was a fatal accident with a Bell 212 in the Maldives some years ago where one causal factor was (it is believed) the co-pilot jettisoning a door early and it took out the tail rotor.
There is some merit to opening a door prior to a 'controlled' or 'anticipated' ditching but not jettisoning it unless in a controlled hover.