Yes, if it was an inside barometer, it could be ranged from 800 to 1500 mb. When diving, the walls of the boat would be contracted by the outside pressure so it would create an increase in pressure on the inside (the state equations in thermodynamics: pressure will rise if volume will shrink in a system where all other parameters are kept constant). I don't know if the local pressure (inside presure) would get up to as much as 1500 mb. That's almoast 1.5 normal atmospheres...but maybe it did raise to that value. One thing I am sure of is the rising in pressure for very short bursts during depth-charge blows. Any explosion is a fast traveling pressure front. It travels with the speed of sound (which in water is 4 times more than in air) and it depletes its energy by moving matter. Most of a depth charge's explosion energy was used on moving the whole boat, but some of it travelled throw the hull and entered the submarine and, for very short periods of time it would raise the presure beyond confort limits (example: the sound of a depth charge explosion would enter the submarine and disconfort the crew with its over 80 d.