for those not fully versed in zoology...medusa = jellyfish
not sure if they exist. May want to check Stormwrack possibly. If not, make something up. I thought they used to have stats for these back in the 1-2ed days. I'd go mostly with Strength poisons, but for some like the Sea Wasp, a Con poison would definitely be applicable. Heck....in an aquatic environment, a Str poison would be the death of something that can't breathe water...
EDIT:
Creature Collection 2 has a Blood Sea Jellyfish.
Monsterous Compendium Annual 2 - Jellyfish, Giant - Portugese Man-o-War
Mystara Appendix - Jellyfish, Giant - Maurader, Deaths Head and Galley
Tome of Horrors 1 - Monstrous Jellyfish
Stormwrack - Jellyfish Swarm