Depends on a lot of variables. If you sent out a distress signal before impact, you will probably be located pretty quick, so dehydration may not be an issue and may be irrelevant. And how likely are you to land close to a clear, trickling mountain brook?

Historical instance- a Navy dive bomber was engaged in a search on the Channel Islands for a missing plane and developed engine problems and had to ditch offshore near Santa Cruz Island. The three crew got out and made it to the nearby beach. They were picked up and home in time for dinner...They did indeed spread a parachute and activate distress flares. This was 1954. Today the sunken plane is a great scuba dive destination.

I got on a bit of a rant because in situations I have faced, dehydration has been a bigger hazard than water borne illness. Of course it is pretty easy to carry means of treating water, so that the issue should never arise....