Gives them constant effect water breathing, the spell was dumb.
Gives them constant effect swift swim 30pts.
Gives them immunity to common disease and poison.
Gives them 50% resistance to blight disease.

I am aware of the temple quest in which you need to drown yourself, a workaround is to damage your health by some other means like casting destruction spells on self or letting a summon wail on you.

The head not showing up in the screenshots is a problem with my game, not with this mod.