If you have raw meat on your hot bar they will follow you a bit longer. If you have a trap under water just be patient. Doesnt need a roof just stay low and he will come to you. It will always go down but as soon as you swim up he will launch up just as fast. 150 first time taming. Good luck as long as your patient and have scuba your good to go.
Go to herbivore island (The island) at the right bottom. Go to map if unsure. Look around and you should find some itchyosaurus. But be careful, in the water they look like sharks, don’t mistaken them as sharks they have a pointed snout. Like if you found it helpful! :)