I didn't notice this when I first looked at that file, but it certainly explains the relative rarity of turtles. Reefs aren't that common, and turtles only have a small chance of appearing on them. I'm not sure what the solution would be as I don't think I would want more reefs because they limit building on coastal tiles and can't be cleared. Although it wouldn't be hard to create a simple mod to do this if the number of reefs were to be increased. I also don't think that I'd want turtles to be able to spawn on coastal tiles either.