Always be looking around your surroundings in the water. If you find a pack it's better to lead the to coast and tame them. Leave tame in water on passive and shoot at them. Friend and I kill them all no matter what this way. If you have a please my luck was to get the hell away and wait till the eels sync up their flee pattern and whistle follow and flee as fast as possible. I use a spino most times but if you can't kill then bolt. Wear riot. If you're fighting on coast have stimulant in hot bar too.DO NOT stand on tame when you're being attacked they shock tame AND you. My luck when this happens is then I'm constantly stunned on back of Dino and die. If you're in the middle of ocean but at surface (You died but spino swam up to surface and is being shocked still) DO NOT fly close to water. Especially on a dragon. You will get shocked off and dropped into the water where they're literally (in my experience) ignoring your dinos and waiting for you to drop into their swarm.
I found one of these and sudenly there were a lot of them around me... They just melted my Megalodon in a second and there was nothing I could do because of the electric shocks!
I advise you, If you arent well geared up and with a Basilo (which is immune to the shocks) please just try to leave this beasts alone, DONT TRY TO FIGHT THEM UNLESS YOU KNOW WHAT YOU ARE DOING!