How deep did these submarines go?

The prewar classes of these submarines were tested to a depth of 250 feet. This included the Porpoise, Perch, Salmon, Sargo and Tambor classes. The boats the U. S. built just before and during the war did go deeper. The Gato class was tested to 300 feet. The Balao and Tench classes were tested to about 400 feet.

Test depth was not the same as design depth. Design depth would be about 50% deeper. In other words, the design depth of the Gato boats would be 450 feet. For the Balao and Tench boats, it would be 600 feet. Fortunately, the design depth was a conservative estimate. Many of these boats would go deeper than design depth, often unwillingly. Fortunately, the boats were built well and were often able to survive the greater depth. Of course, we only know about the boats that made it back to the surface.