Try and stay above them in the water column so they have a hard time hitting you. Then stab the hell out of them with a sword or make your way to shore/ a rock. - These dudes are nasty. They need to be knocked down a notch. I stopped to check the level of another Ichthyosaurus and around 5 of them came out of the coral bed and swarmed me. I barely escaped with my current Ichthy one hit from death.