mvincent You still appear to not be getting it. The premise was that the core rules did not explicitly [I said:forbid[/I] flying from being used underwater, so that any supplement that presented new rules for doing so would not be invalidated.
You still appear to not be getting it. The false premise was that the core rules did not explicitly forbid flying from being used underwater.
In reality, the rules explicitly forbid flying underwater by not putting underwater on the allowed list of environments where a fly speed works.
It is not an explicit exclusion (like you appear to be wanting), it is an explicit lack of inclusion. It is still explicit. Doing it your way would require that they list all environments that are excluded every time for every movement mode. What a waste of space!
Any supplement that presented new rules for doing so would still be a change to core rules because the core rules are still explicit here.
So, your point is invalid. It proves nothing other than the fact that you are not getting it that there are indeed explicit limiting rules here and that your conclusion concerning supplemental material is based off an initial faulty and invalid premise.