Dismount your Dino when ambushed by jellyfish, whistle follow then swim down deeper. After the initial shock wears off the Dino finds it easier to escape downward unmounted. Don't necessarily wait on it either, swim to a safe distance and it will catch up.
Use the ichthy they said... it's immune to shocks they said... that's fine I'll quickly swim done with no scuba gear and lure one to the surface...zap... surface is so near yet so far... manta finish the job
While your tame still might die anyway if you have a high hp dino that gets zapped by these whistle followall and swim away. If you can i managed to rescue my 140 rex that way. She had 8k health and lost around half with a normal saddle. I came back with my re and killed them with rock throw. (It was super effective)
Using a quetzal, kill the ones on the surface for easy bio toxin, then use it to make shocking darts, which are better than tranq darts, or use the bio toxin as a better narcotic for being Dino's torpor up
They will kill you off faster than the Shining, and the can also paralyze your Dino and you. I wouldn't use a water Dino, I'd find one, check if any others are there, and then I'd use Crossbow and good tranqs.