Hmmm, then sea turtles should kosher - they have fins and scales.
I've never thought of that.
As I understand it, there's a statement somewhere in the Talmud that "there are fish [water animals] with fins and no scales, but there are no fish with scales and no fins." Sea turtles would seem to go against that. But then, so would sea snakes.
I'd check with a poseq before I ate anything!