dcollins said:
Yep, there was a huge argument last time this topic came up. I'm on the side that does not like allowing it, because it lets fly (3rd level) make freedom of movement (4th level) basically not useful.
You'll note that in order to make sense of the "yes" side of the argument, Artoomis, above, had to distinguish between "natural" flight and "magical" flight -- or else creatures like dragons with flight, and slower swim speeds, suddenly don't make sense. But, this distinction does not actually appear in the core rules, nor is it defined for any monster with flight.
Freedom of movement is still quite useful.
1. You can move normally - that is, at full normal speed, or, if flying, at full flying speed.
2. You can attack normally - allowing the spell "fly" to work underwater does not remove any attack penalties.
3. It works on ANY magical effect that impedes movement - underwater movement is not its prime purpose.
If ALL you want is underwater movement, though, I'd allow a spell caster to research a new spell "Underwater Free Movement" (or some clver name, perhaps) that only affected underwater movement. Probably only second level - maybe even first.
By the way - I'd allow underwater flight for any creature that has an ability similar to the "fly" spell. Like, for example, a beholder (well, maybe - unless you view it's flight to be tied to a "gas" type effect - then it can't go underwater because it relies on being lighter-than-air, and thus floats on the surface) It just takes a little common sense for what you allow.
Frankly, though, creatures who don't normally go in the water have many reasons to stay out of the water, so I wouldn't expect that to be much of an issue.