i haven't heard of the RXPs coming with a depth gauge from the dealer.. my 06 GTX has one from the factory..
i do have to warn you.. most of the time when your traveling at cruising speeds and you notice your getting into shallow water, it is most likely too late..
some more food for thought, the OEM finder is not the best in the world, i sometimes see 0ft when im fiding in areas that i know have a much greater depth.. if found it to work most consistently when at a stand still.