Where did the water go when submerging the boat?

Most of the water went into the main ballast tanks. These saddle tanks were outside of the pressure hull. Normal diving filled the main ballast tanks completely. The tanks were open at the bottom so that, when the vents were opened, the water flowed in quickly. Main ballast tanks were not subject to sea pressure since they are open on the bottom. Sea pressure equalized as the depth changed.

Some dives did not use all of the ballast tanks. For example, the captain might only want to go to “radar depth”. In other words, he wanted to be able to use the radar while showing as little of the boat as possible. As a result, he didn’t want the boat fully submerged.The radar had to be out of the water so it was still operational. The captain might also just want the decks awash for some operations. In these cases, some ballast tanks would remain dry or only partially filled.