Don't recommend swimming close unless you have a Basilasaurus, lost my favorite itchyasaurus, her name was flipper , but I learned if you ride a raft, you can get close and stab with spear, easy removal of killer jellyfish.
If a cnidaria killed my spino, boy i would be mad. i lost my sarco to cnidarias, a scare with my icthy with cnidarias, and almost got my megalceros killed by some. ( he is dead now D:) I kill any of these that i can. The dossier says that they are passive, but aggresive when approched, NO! as soon as i step in the water these attack, and they litterally don't move at all, they just sit there and chill until you or a tame steps in the water. one like=one prayer for my deceased sarco, Gatorade (you can find his story in sarco) and one member of the cnidaria hate club.
I think that the devs need to reconsider some of the creatures in ark. With the jellies and electrophorus, it's too much. Especially on fjordur. There are hundreds of eels EVERYWHERE in the shallows! I don't mean reconsider as removal, but they need to be nerfed. No person I know tames mosas, plessies, or dunkies anymore because of those two creatures. They've made a lot of ocean creatures irrelevant. My first water creature is always a basilo. And that's all I need, because what's the point of anything else.