down&out
I have seen photos of ditched single engined light aircraft and the front (the engine) submerges almost immediately, sometimes even lifting the tail out of the water for a short while, so (in an unpressurised i.e. unsealed aircraft) I would expect the cockpit to fill up from the area of the pedals and the wing roots first.
A low wing aircraft ought to give you more time, as well as and more crucially giving you somewhere to stand while messing about with the liferaft.
But
AFAIK the statistics on ditching don't appear to significantly favour low wing aircraft.