For Mobile ultimate you have to be dragging a fish and have fish meat in your last inventory slot. It is a passive tame, but won’t let you feed the otter the fish meat in your inventory unless you are dragging a dead one. I highly recommend scuba flippers and tank for following it around on an official server (since you cannot carry it with a tame). I found two in the pool just East of the redwoods hanging around a beaver and its dam.