These turtles are very strong. Many years ago as a young man, I decided to once again show-off to a group of divers by pulling a large sleeping loggerhead out from under his ledge. Normally the turtles would try to swim away and I would ride it for 30 or 40 seconds without seeming to bother the turtle too much. They are strong enough to pull a scuba diver faster than you can sprint underwater. Using your fins and a lot of muscle you can normally steer them in any direction and use them as an underwater scooter. When released, the turtles would usually take off, sometimes head for the surface and other times head back toward the reef and sometimes just go back under the exact same ledge.
However, this particular turtle was much different. Upon being forcibly removed from his ledge, he immediately and deliberately swam so as to flip me over onto my back. Simultaneously he was extending his neck backward and to the side and was aggressively trying to bite me. A bite from these turtles would be a serious injury.
Luckily, I had one hand on the shell immediately behind the center of his neck and the other was back under his tail. He was unable to bite me, but had flipped me completely over on the bottom and the back of my head was being pounded into a bloody mess by contact with sharp coral and rocks. Basically we were both upside down with the turtle on top.
I was unable to let the turtle go because he was making every effort to flip over and bite me and the only defense I had was to retain my hold on his shell. I was struggling to regain control and get on top of him and to stop the abuse to my head. Finally after a very long minute or so, I was able to wrestle/swim the turtle into a position below me instead of on top of me. I then tried to throw him away from me into the water column as I backpedaled as fast as possible. When released, he stopped himself and took an unusual vertical position and was clearly very agitated. He spun slowly in a 360 degree turn and I think he was trying to determine which of the divers in our group to bite. We all backpedaled and sculled with our hands backwards very quickly and he took off.
That was my last experience with riding loggerheads with SCUBA gear, Clearly it is a dangerous and stupid activity. Also, I'm pretty sure it is illegal to harass sea turtles in this manner.
They are normally very docile and sometimes a little curious. I have heard reliable stories of them trying to mount scuba divers during the mating season, but this must be a very unusual occurrence because we commonly see 3 or 4 turtles on nearly every reef dive in the summer and I've never seen a problem with turtles unless you bother them.