[Hitman]

A submarine is much like a balloon, perfect balance is theoretically possible but almost impossible in normal conditions. You are always reacting to the boat raising or sinking, and the most you can pretend to get is a quite steady condition, but some minutes later it will have sunk or risen.

There is one exception, though, and that is finding a thermal layer. Because the colder water below is more dense, you can actually have the boat rest on the "layer" that separates both areas by trimming it heavier than the upper one, but lighter than the lower one. It will not break the "surface tension", but you need a pretty heavy difference thermal layer to do that. This tactic is not unusually used in modern diesel-electrics and nukes, albeit at greater depths.