Make a stone or metal box underwater, two walls high and three or more walls wide. They flee if you use any narcotic arrows or tranq darts so avoid those when kiting one into the box. Once inside, they cannot escape through a gap one wall thick or high.
Very easy to tame if you know what your doing. First tame an Ichthyosaur. Then grab a crossbow and some tranq arrows and find one and knock it out. It is not be able to attack if you are on the Ichthy. As you are taming it, be sure to remain on the Ichthy unless feeding narcs, so other creature don’t hurt you.