The Navimate was a hot subject 3 or 4 years ago at the big dive equipment shows around the country. They had some pretty slick mockups but never a working model. As others have said, gps signals won’t penetrate water very well. Navimate was supposed to employ some hyper sensitive electronics and proprietary software to detect the weak signals. Another problem they needed to overcome was gps signals travel in waves like any other radio signal and are refracted when traveling from air to water which would cause an error in actual position under the water.
Looks cool....I hope they make it work someday!