I think it's mostly batteries, a sub operating at constant depth and in motion is negatively buoyant. If something goes wrong and it loses propulsion, power has to be available immediately to operate the systems that can handle the emergency - whether it's blowing ballast or restarting the reactor. This is a somewhat different constraint than the procedure described by the other reply - ships float without power, subs, not always.

I mentioned this upthread - USS Thresher[1] seems to have suffered a loss of both propulsion and ballast control while near test-depth with, unfortunately, tragic results for the vessel and all on board.

[1] http://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)