Very easy tame. I used a plesiosaur with about 11k health on official pve servers, but a mosa or megalodon should work. Depends on the saddle but I lost about 2k health per feed, so more health cant hurt.

Make sure your dino is on passive and following you.

Swim down to the tuso and let it grab you. Jump off and swim towards the base of its tentacles. Feed the beak.

Then swim up as fast as possible. Get out of render and the tuso should let go. Once in render again the dino should follow you but the tuso will stay down. May take a couple tries as the tuso can grab again.

Wait 14 mins and repeat.