An escalation of tensions during the latter part of the 1930's indicating the possibility of a war in the Pacific led the US Navy to actively develop a submarine capable of supporting surface fleet operations. Since
Submarines were originally looked upon to serve as scouts and screens for the navy's capital warships; the desired "fleet boat" would be required to possess the speed, the range and the ability to maintain extended deployments which would be necessary for a war in the world's largest ocean.
This design evolved slowly during pre war years that culminated with the Tambor and gar class at the outbreak of war. Two more designs were evolved during the war. The Balao and Tench.
The evolution of design is as follows:
S Class - WW1 submarine with 4 bow torpedos
Porpoise Class - 4 bow, and 2 aft torpedos
Salmon Class - 4 bow, and 4 aft tubes
Sargo Class - a refinement of the Salmon class, with the same number of tubes
Tambor/Gar Class - The first fleet boat that had the specifications the Submarine service was looking for with 6 bow and 4 stern torpedo tubes.
Gato Class - This was the "standard" fleet submarine of WWII. A refinement of the Tambor and Gar class, with the same number of torpedo tubes
Balao Class - The Balao class was an upgraded version of the Gato class, mostly noted for increased diving depth.
Tench Class - a further refinement of the Balao.
Most don't realize, but some of the Gato, and many of the Balao and Tench boats served well into the cold war. The last one being decommissioned in 1972 if i remember correctly.
As for shallow crush depths.... give this thread a read.
http://forum.kickinbak.com/viewtopic.php?f=29&t=414