Corrected entry: When the barracks are completely flooded, Eggsy breaks the two-way mirror with his fist, and only a few hundred gallons of water pours into Merlin's observation room, spilling the trainees onto the floor. A moment later, the floor is merely moist, and Merlin is apparently dry. If the watery contents of the barracks (flooded 9 feet deep) had suddenly gushed through the window, it would have completely flooded the observation room to several feet in depth in a split-second, and with lethal explosive force.
Charles Austin Miller
11th Sep 2015
Correction: In the scene, the room actually has a large gap between the floor and the walls, which most likely leads to drains to keep the water from flooding the room.