Because that doesn't give you a big hero reveal with accompanying musical score that makes fanboys go 'twee!!!' Rob+
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.
Hmm. The ENT and DS9 ships were being crushed by the pressure of immersion in a fluid. The TOS ship was in the grip of a pseudo-physical hand; the mechanical pressure created by that thing was uneven and thus no doubt far more dangerous to the structure of the ship, as it would create twisting and bending rather than evenly crushing forces. OTOH, the ENT ship was damaged to start with, but then again, so was the DS9 one. Difficult to say which would best represent the "starship norm", but it does seem logical to assume that the Defiant was the closest to being undamaged and in water-analogous circumstances. While she's explicitly more heavily adorned with physical armor than the average starship, the order of magnitude would still hover around the realistic (indeed, found in the real world of combat submarines!) megapascals. And that's before SIFs, supposedly, as those have been shown to work real wonders against bending or crushing stress in collisions. But yeah, lots of room for speculation. Timo Saloniemi