Depending on how high it was initially floating in the water, I'm amazed they managed to get that door open against the pressure of the water outside.
Not really. As soon as the handle is rotated, the door actually moves in (in this case it would be helped to do so by the water pressure) and exposes gaps all around the perimeter.
With the gap at the bottom and lower sides, water would enter and the pressure soon equalise - more or less.
An old 4WD piece of common sense in river crossings if things go pear shaped and you need to get out - let the water in first so that you are not trying to open the door against the water pressure. In the car it is a case of opening a window to achieve this, then evacuate. Getting the door off the stops in a 737 would have the same effect.