If the piranhas they say are some of the most annoying, these bastards are deadly af. Mantas pierce through armour, shred it, while these guys stun you. Include the piranhas, and omae wa mou shindeiru. Best bet is Basilosaurus, or decent levelled mosa ( not recommended as shock attacks are the weakness of the mosa) and tusoteuthis, because of the attack range. I hunt these down to total extinction, then gladly save up the bio toxin they drop for giga, royal griffin ( I play on mobile so...) or taming of other whatnot thingys. If you want to kill these early game, as they are seen on the shore quite easily, just grab a few spears and/or pikes then slaughter these bastards. Lure them by walking until swim mode into the water, then depending on your movement speed go backpedal to the shoreline. This will successfully lure them near you and you can slaughter them as you please without endangering you or your mount.
These things spawn on The Island at the bottom right of the map near the Lava Cave. These cheeky little buggers are more than happy to ruin your day by swarming and stunlocking you and your tames nonstop unless you are on a Basilosaurus, which is the only creature that is immune to their attacks. If you don't have a Basilosaurus, lure them close to shore and cheese it with a spear or pike, better yet pray that they beach themselves and die.
The Cnidaria is a small jellyfish found in the oceans. This easy-to-spot creature won't attack unless approached too closely. It delivers a sedative sting that can knock out enemies if striked repeatedly. While it cannot be tamed, survivors will hunt it to gather its sedative, which is used for advanced tranquilizers.