My understanding is that the pressure below the surface of water is pgh.
p - density of water, g - accel of gravity, h - dist below surface
And that this relationship holds regardless of the shape of the container of water. (also, I'm neglecting the atmospheric pressure at the surface)...
Imagine support C is not there. Imagine each boy has walked for 1 second. Each boy exerts a moment around B equal to is weight times his distance from B. If BoyY's moment is larger, the plank will want to rotate CW. But the plank doesn't rotate because support C is there. Will BoyX's moment...