What does this spell actually do? I don't have a helpfile to work with.

Does it make a person lose mvs while they fight, or does that only happen under water?

Does it completely nerf bash, or just reduce it's chance of happening/efficacy? Any other skills?

BUOYANCY 'MASS BUOYANCY'
Syntax: cast 'buoyancy'
cast 'mass buoyancy'

This spell sharply reduces the mass of the victim for a very short instance.
The major effect of this is that forceful physical blows may throw them
away from their current location. This spell is considerably more effective
on flying victims, who are less able to maintain contact with surfaces to
slow their movements. Moving under water is exceptionally fatiguing under
these circumstances as well, given the difficulty of forcing it out of your
way. Skills that require a great deal of momentum will be difficult or
impossible for a Buoyant individual, and these skills will also be more
effective against such a victim. Buoyant victims will have difficulty
finding sufficient leverage behind attacks with weapons.

This spell requires the trust of a victim if it is to be cast without
resistance, though it may be forced onto a hostile victim if they cannot
resist it.

Mass Buoyancy is always hostile, and attempts to render the victim's entire
group buoyant.