just west of the castle in the water is where all the sea turtles hang. Unfortunately visibility stinks underwater so you often end up with adds on a pull. I pull with snare then if i have one still biting me I know I have an add. Get out of the water and run far before snaring the add and the original one will go home.
The ancient i found laying on the bottom whilst the venerable was floating near top like typical turtle. Did not seem special other than name and loot.
Have never looted anything at all off the regular turtles. They snare and fear easily so I use them to train archery and 2hs.