Well, just to be nit-picky, “the middle” would be irrelevant here, as water pressure is dependent on depth, but not position (or total water volume for that matter). So anywhere at the lowest depth would give you the best surrounding pressure.
The only logic in which I can actually see this survivability idea working is with the assumption that the attack would be from a normal cold war ICBM. It might survive an air burst, and a dam would represent a rather difficult target for a ground burst to hit. The accuracy of ICBMs is far poorer than some seem to think. 
Of course the reservoir would be a pretty easy target, and a ground burst there might be enough anyway.