"A battery operated, hand held Garmin Gps device "
...only works on the water surface. The ship has a better one, but it won't tell us where the ROV is , other than in very general terms (those are already known). The ROV uses an underwater transponder system to keep track of it's location in 3-D. A computer system combines the surface GPS location data with the underwater transponder location data to figure out the ROV location.
I don't know what sort of transponder the ROV used, other than what is in the Finding Amelia film. I'm a little familiar with two types - an acoustic one (sort of old "sonar" technology), and a variety of radio-based ones, mostly using relatively low frequencies with an underwater antenna (that's my guess on what got chopped by the prop). The ROV picks up the signals from the antenna, in a way that resembles the way a GPS picks up the signals from satellites, but they are not the same signals or frequencies.
Later edit: Seabotix has a nice website worth visiting to learn more about ROVs.
http://www.seabotix.com/products/tracking.htmThe most obvious backup system would be to use a surface ship with a commercial sonar "fish finder", hovering over the ROV location. That's dangerous duty in the surf visible in the video.
Finding any object on the reef that can be identified as belonging to a Lockheed might be the best way to attract investors. Fuzzy videos without scale aren't enough. We might be fooling ourselves when we identify tailwheel assemblies. What if the black squiggly thing is a biological growth 12 inches tall? Then the "tailwheel " feature couldn't be the size of an Electra unit.