The water gets in because the winscreen shatters when the nosewheel (mainwheels in a taildragger) drags the nose down and the thin perspex windscreen isn't designed to resist water hitting it at 40+mph so it fails.
I've been told that most taildraggers flip over on ditching whereas trikes tend to plough in as the drag of the wheel is better spread along the heavy end of the aeroplane. Anybody got any examples to back up / disprove this?