Note I said \'kept the pressure close to sea level\'.

In actuality the pressure in a U-boat would normally climb during submerged operations. This was the consequence of bleeding air from various operations (such as ballast tank filling) into the boat hull. This was done rather than let the air displaced by a filling tank bubble to the surface, marking one\'s position and making a lot of noise.

Even after prolonged submerging, this pressure rise was too slight to give anyone the bends - only a few psi at most.

This did play subtle havoc with the torpedoes, whose depth sensing mechanism used a \'sea level\' reference pressure diaphram: after prolonged exposure to submerged operations the pressure in the diaphram leaked enough to increase beyond sea level, making the torpedo go deeper than set. Since torpedoes were normally tested during development from the surface, or from subs submerged on a one day test run, not a multi-week cruise, this flaw escaped detection till discovered in combat. This problem was found, and fixed, by the British in WWI, then forgotten and rediscovered by both the Americans and the Germans in WWII, who both developed this \'pressure malady\' in their torpedoes. You\'d think someone would have remembered.

Submarine hatches were designed to close against outside pressure (ie: opened outwards, so external sea level pressure helping to close, and seal, the hatch), the normal situation when submerged. If you surfaced with higher pressure inside the boat than the outside atmosphere, the higher internal pressure inside the boat would aid the opening of the hatch - rather than be impossible to open, it would be easier, even fly out of your hands. There were several instances of U-boats opening their conning tower hatches in these situations were the hatch would fly open, and the opener, usually the commander, being literally flung out of the sub by the release of air from the hull. Several injuries occurred from this.