When I was going my OW checkouts in San Soloman springs, this inch and a half tetra took an interest in my ear. Now these are of the same family as Neon Tetras, characins, but also the same family as Piranhas. Bloody bull-dog jaws! Bit one ear, then the other; hurt a lot. I'm trying to pay attention to the class, covering my ears, then explain to the DM who thought I couldn't equalize - in sign language: "friggin fish biting ear!" ROSLTR (rolling on sand, laughing through regulator)
I always snicker at those who ocean dive without full suits, as they're the ones who need the vinegar I carry, but once I did a G-Stride into a Sea Nettle. Ouch. After I finished the dive, it hurt a lot: Vinegar, followed by Calamine, followed by Sudafed + Antihistimine. Seemed to help. Today, I always wear a beanie, too.
Okay - Sara3 was looking for serious answers. How about this:
(1) Always wear a long suit, and maybe a beanie.
(2) Don't touch anything, ever, for any reason. Details on request.
(3) Avoid jellies until you learn which are harmelss, use octo to purge a path through them on ascent, and really avoid Porteguez-Man-Of-Wars (I've seen them twice in 140 dives.)
(4) Ask captain, DM, LDS about local hazards. Other side of the world has more dangerous critters than this side, i.e. blue-ring octopus, deadly snails, dangerous lion fishes (yeah, some get released over here when they outgrow aquariums). There are few, but it doesn't hurt to ask.
Marine animal injuries are very rare, but I always carry 4 oz each vinegar, alcohol, calamine, and some generic Claratin D.
No worries, mon

have fun,
don