I've always thought of the dragon turtle less as a "monster" and more as a "force of nature" kind of creature. It appears, attacks a ship, either destroying it, or badly damaging it and devouring some of the crew/passengers, and then departing. Leaving the PCs to deal with the wreckage in its wake.
I would make it bigger (lots more HP), give it the ability to do substantial structural damage to ships instead of just a capsize, and get rid of the silly superheated steam ability in return for swallow whole, maybe some sort of water jet to knock sailors out of rigging, and maybe a massive, snapping-turtle-type bite. And it should have some reach. Turtles actually have fairly long necks and can extend their heads quite a ways out of the shell.
It should be able to come up from below, deliver a ship-cracking blow, grab a victim or three, and sink below again without ever being clearly seen and without exposing itself. Two or three passes should leave all but the largest ships with broken keels and taking on water fast.