Turtle Inn is about as upscale as it gets in Placencia. I've never stayed there, but I've had breakfast there a couple of times. Me, Charlie Sheen and Julia Roberts all sat together--ok, just kidding. I've never seen a "celebrity" at TI although I wasn't really looking for one.
Having said that, I'm trying to remember ever seeing a dive boat with the Turtle Inn logo on it. Vaguely, I guess.
About the only thing I've heard about TI is the story, not sure if it's true and don't remember who told me, about an Italian millionaire and his family staying there. Much to their surprise, the cabanas weren't air conditioned and after spending one hot night they cancelled the rest of their stay and moved on to a lesser hotel that offered a/c.
It's close enough to the airstrip that you could walk it in just a couple of minutes, but I'm sure they pick up their guests. This means it would be a bit of a hike to go into the village, but I see their van(s) running guests in and out all the time.
I would think that if they have their own in-house dive op, it would be well-run. I doubt Mr. Ford-Copola would allow anything less. I've been a member of this board for a number of years and don't remember ever reading a TI dive op review. If you go, please write one up and post it.