Cleon
Legend
Frankly, I'm more in favor of just a magical beast version.
Well, the AD&D monster we're converting was a non-magical creature, so it'd feel odd not to consider a mundane set of stats.
There's just not so much different here compared to the other big turtles. However, if you really want to do it, let's go with what you have and maybe just give it a Hide bonus in water. Unless you mean it camouflages itself as an island.
Why, yes I did, or possible a "blend in to the ocean" ability like we gave the Globigerina:
Transparent (Ex): A globigerina is difficult to see when it is in the ocean, since it blends in to the color of deep water. It takes a DC 13 Spot check to notice one. Creatures who fail to notice a globigerina and move within 5 feet of it are automatically attacked.
Something like this...
Camouflage (Ex) [or Mimicry?]: A giant crypto-chelonian can blend into the color of deep water or imitate the shape of a natural marine feature of approximately its own size and shape, such as a small shoal, an islet, or a mound upon a beach. So long as the chelonian does not move, it requires a DC X Spot check for another creature to notice it is not part of the scenery. It takes a cypto-chelonian 1 minute to assume a camouflage.