Just build a taming pen.
3 Behemoth Gate Frames are prefect (2 might work as well) + 2 Gates
Close one, the other one should be open.
Ride an Ichty and let it follow you.
If you done it correctly it can't get out and due to it's terrible turning radius you can swim with your Ichty to the gate and close ir
Center tip: as they have added the mosa to the center I've found the easiest way to knock one out is find one close to the bubbles get its Agro and swim in the bubble the mosa will get stuck and it can't hit you. Also this gives you unlimited oxygen and sanctuary from being attacked by anything else. ðŸ¤˜ðŸ¼