A passively tamed creature that requires you to,”Feed” it a school of parakeet fish. (Tiny glowing schools of blue fish that look like drones, usually around the turtles themselves.)
You need to aggro the school of fish by walking into them which gives you a,”You are being targeted by a Microbe Swarm” pop up, then slowly lead the school to the turtle and swim alongside it which automatically starts the taming progress at 0% which then slowly climbs to 100%. During the taming process Sabertooth salmons and coelacanth will randomly spawn around the turtle and try to kill the school of fish to stop the tame.
However this can be easily cheesed by killing all the salmons and bigger coelacanths until a small coelacanth spawns which cannot keep up with the turtle and the school of fish, so let him try to catch up while the taming bar fills up.
Scuba gear is also not necessary, as the turtle constantly produces oxygen around it which refills your oxygen meter.