I did some calculations with the given pressure values on http://en.memory-alpha.org/wiki/GSC
and an approximated gravitational acceleration on Earth of 10 m/s².
The Klingon Raptor-Class can withstand a pressure of 15000 GSC (g/cm²). On Earth this would be about 1,500,000 Pa. This is the pressure in 140 m (460 ft) deep water.
On Kirk's Enterprise it is getting critical at 1000 GSC (that's nearly the standard atmosphere pressure). Told exaggerated, a normal anticyclone could be a problem for the Enterprise. So there is no way they could dive underwater.
Sisko's Defiant can even withstand 9,000,000 GSC (900,000,000 Pa or 9000 bar). Theoretically they could dive down to 90 km (56 miles).
So these are three widely differing canon values with a lot of room for speculation.