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.
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.