The 737 (or so Boeing say) should sit tail heavy in the water partially submerging the aft doors.
BA have the same policy - and the front doors should only be opened once it has been safely established by the crew that they are above the waterline (ie crew are supposed to re-enter from the over-wing exits if they see it is okay to do so and that the front doors are above water). Hence the reason not to instruct pax to do it straight away.
So far as I am aware it is standard for the over-wing exits to be used as the primary exits in a ditching on the 737.