A submarine is stranded on the bottom of the ocean with its hatch

Using sea water density of 1,025kg/m^3:

Pressure above hatch = (1,025 x g x 25m) = 251,125Pa. (g = 9.8 used).

Neglect the pressure inside the sub, because it is equal to the air pressure at the surface.

Area of hatch = (pi r^2) = pi(0.225^2) = 0.1591m^2.

Force required to open hatch = (251,125 x 0.1591) = 39,954N., or 39.954kN., approx.

Not much. You flood the escape chamber first by opening a valve, so when you open the outer hatch the pressure is the same both sides.

But if you don’t know that and try and open a hatch outwards, then the pressure at depth h is rgh where r is the water density and g is the acceleration due to gravity. Plus atmospheric pressure at the surface, so that cancels out. Then force = pressure x area, area is pi*r^2.

Something like 4 tonnes force.