It will take around 10ish minutes to knock a high lvl out. You must bring 2.5x the arrows it says will knock it out and extra crossbows. Extra scuba tanks are also recommended since you'll be in the water for quite awhile and the first will most likely break. Once you start shooting it it will try to flee but is easy to keep up with. This is why you must clear out the area around it and keep it in the area or you might be lead to predators. It will take you on a journey if you don't corral it in an area. Occasionally it will turn around and take a bite out of you and your tame then will go back to fleeing so be vigilant. After doing all this, just keep firing away until it finally goes down.
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.