The only reference I can find on water and incorporeal creates at all is (emphasis mine)
An incorporeal creature’s attacks pass through (ignore) natural armor, armor, and shields, although deflection bonuses and force effects (such as mage armor) work normally against it. Incorporeal creatures pass through and operate in water as easily as they do in air. Incorporeal creatures cannot fall or take falling damage. Incorporeal creatures cannot make trip or grapple attacks, nor can they be tripped or grappled.
This is in a section on attacks, but it's phrased very generically. Given that this creature doesn't have a fly speed, the way I read this RAW is that it can maneuver horizontally at land speed without Swim checks, but that it can't maneuver vertically.
How fast and in what directions can a flightless incorporeal creature move in water? Are there any other relevant rules that help to clarify how a flightless incorporeal creature should interact with deep water?
RAW is preferred if it exists, but my suspicion is that it isn't specified.