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 suck and are extremely hard to kill, and will kill any dino near it. However, if you lead it to shore and continue to punch it or use a weapon on it while it is stuck on the shore, you get a lot of biotoxin xD