How maneuverable is a Spiritualist's incorporeal phantom in deep water?

Question

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.

Answer 1

Unfortunately, you are walking into a confusing mess that Paizo has not clarified, despite requests for clarification.

Incorporeal creatures simply do not interact with their physical surroundings except through magic. Every single incorporeal creature in the game can fly, and can do so through any sort of physical medium—air, water, solid stone, and so on. Because physical limitations like gravity, flow, and barriers just don’t apply to them unless they are magical—and not even always then.

Every single incorporeal creature save one—the spiritualist’s phantom.

Originally, the phantom was simply statted with a flight speed, like every other incorporeal creature. At some point, that “feature” was moved, absurdly, to 9th level. The presumed reasoning for this change is balance, though I think they were fighting a losing battle when the thing was already incorporeal. That’s neither here nor there. What’s important is that they made this change, and then did not explain what it means.

Does the phantom immediately fall to the center of the planet they are standing on, being able to pass right through all the solid (and/or liquid) material between them and it, but not, apparently, having any ability to resist gravity? Presumably not, but that is what the rules would leave us with at this point.

So basically, everything about this is left up to your GM to figure out. You’ll have to ask them, because Paizo apparently isn’t going to tell you and without more information from them, we can’t.