In January I swam from Xtabi to Rockhouse.
I ran into something on the way back - I had lap-swimming goggles on not a mask.
Still I'm pretty sure it wasn't a jellyfish, but when the crew at Xtabi looked at the bumpy rash they said jellyfish.
I remember it started burning right after I lifted the rope by Rockhouse (to swim under the rope) and the rope brushed my forearm (where the rash was).

Anybody got any guesses other than jellyfish?