Chris,
I apologize. Im sorry I missed answering this a couple of weeks ago!
Properly cured resin is safe to wear. I dont know that body heat will do much to impact the resin, but leaving it in bright, hot sunshine (or something equivalent) will.
Making a resin bead should also be safe for hair.
I dont know as though I would call resin castings waterproof. While getting them wet (like washing your hands) doesnt concern me, I dont know that I would recommend them for beach wear (sun and salt water) without testing them first.
If I was going to try this, I would definitely use epoxy or polyurethane for this and NOT polyester resin, since polyester will yellow with UV sun exposure.