Not just corpses. The wrecks themselves are well-preserved. It's cold, it's freshwater, and has been navigated for long enough that it is one of the best places on earth to study shipwrecks.

But due to the corpses more recent wrecks are very restricted. Basically if the crew members are in living memory they don't want people exploring the wrecks since the bodies are still recognizable.

Cold water actually holds more suspended oxygen than warm water, but the cold itself does inhibit bacteria that would decompose the bodies, so you're mostly correct.