Fleet boats could go have gone deeper then many people (including the fans) think. You might find this thread interesting:
http://forum.kickinbak.com/viewtopic.php?f=42&t=414
Some good and interesting info there, but to summerize a few talking points:
- USS Salmon (salmon class, duh) went to 520 feet and was holding steady, though sinking slowly.
- USS Gurnard (gato class), is beleived to have passed 700 feet. To quote, "The depth gauge was well up against the limit pegs and the sea pressure gauges used to calculate depths beyond the depth gauges were only designed to calculate a depth of 650 feet."
- USS Tang (Balao) went to 620 feet in a test dive out of sea trials. Is reported to have gone to 700 when damaged, but came back up to 600.
- USS Chopper (Balao), out of control, depth exceeded 1000 feet as mentioned by VC sniper.
Now all that said, many areas of operation in the pacific were shallow, so if one were to draw comparisions to uboats, any depth advantage a uboat would have had is irrelevant in many places.