Water did not spill over from one boiler room into the next. This is a nice but wrong explanation in the movies. Boiler Room No. 5 was filling already from the damage on the starboard side coal bunker. Leading Stoker Barrettt who was in BR 5 gave the time he left as about 1.10 a.m. when the BR started to flood which was more the failure of a coal bunker door which hold the water back in the coal bunker and was not designed to withstand such water pressure.
Barrett used the escape ladder which lead to the port side of E Deck and he was clear that water did not spill over.