I'mnot sure,but I think that the suits/makeup were inspired by the Gillman from the old horror movie, "Creature From The Black Lagoon" from back in the 50's, in which there were extensive underwater scenes. If it was safe way back then, I'm sure the costumes used on "Buffy" were just as safe, if not safer. It looked to me as though the whole thing was just modified wet suits with a few rubber pieces attached.
(For anyone interested, here's the original gill man from back in 1954)
[ame="http://www.youtube.com/watch?v=ariuokNFhSw"]Creature from the Black Lagoon (4/10) Movie CLIP - Underwater Stalking (1954) HD - YouTube[/ame]
Scared the living hell out of me back then.