Infiniti2000 said:
No one did. It's the rule in the plane of air that all flyers improve one category. For the same reason(s) you apply the rules from the water plane, you apply the rules from the air plane (pun intended). From what I understand, Musrum is altering how the rules from the water plane is applied such that he intends to negate allowing the plane of air rule on improved maneuverability. Interestingly, which I didn't see till just now for some reason, the plane of air rule would cancel out the loss in maneuverability for flying underwater.
I'm pretty sure I said this, but I'll repeat it. It's fine to use the flying underwater rule from the water plane as a guideline if you want to allow flying underwater on the prime material. The correlation to hampered movement helps. But, IMO it cannot be claimed as a rule or precedence or anything. It is absolutely against the rules to allow it (not that allowing it is a bad thing*).
* Allowing it without restriction is a bad thing, though, as evidenced by your rebuttals vs. sparrows and the like. You just aren't clear on what the exceptions are because there surely are no rules against it. In other words, what happens to a sparrow (e.g.) on the plane of water?
1. A sparrow on the plane of water drowns.

Assuming some breathe water capability, it'd fly at half speed and down one manueverability class. This matches up nicely to what seabirds actually do when flying underwater (except maybe it's too fast).
2. The plane of air rules have no bearing here whatsoever. Discussion of the plane of air rules is a red herring.
3. The RAW already allows for using one mode of movement in another (less than ideal) medium - walking underwater.
4. The rules provide for a mechanic for "hampered movemenet" but do not specify how it works for air flying - for example, flying in reduced visbility or in really, really thick air.
5. The plane of water rules provide a nice mechanic that can be used for how hampered rules would work for flying underwater.
There you have. All based upon RAW.
By the RAW alone, flying should be allowed underwater, but the RAW actually does not provide the mechanics for how to apply the hampered movement rules to this situation. The plane of water (MotP) rules can be used to fill that gap. You might come up with some other rule, and that would be fine,too, because where RAW lets us down is defining the mechanics for hampered movement for flying. The MotP rule is a great place to start.
Now one could also, per RAW, maintain that you cannot fly underwater at all, but that seems to be both over-literal to one rule and ignores (at least) the precedent of walking underwater as well as comparisons to a number of real-world creatures that fly both above and below the water.
Obvious exception includes gas creatures or others that basically cannot be submerged.
Of course, to be complete, I'll restate that flying underwater does NOT equal swimming.