The wiki sais that you can find them in the deep oceans. However on the island at least i only found them near tge surface or near the shore. Especially around carno island.

I used cheap and expensive items for taming and it seems it makes no difference. I used cactus brew as well (don't forget that you can't eat while swimming, so uncyo and mount your tame to eat when timer runs out between the taming process)

Wen du ihn begegnest er kann springen und das macht Sau viel schaden bzw stark