For free skiing, it affects how fast I am moving relative to the water. A boat traveling at 34 mph on the gps in a 2 mph current feels faster or slower to the skier depending on if the boat is traveling with or against the current. Having a traditional speedo (assuming I calibrated it in open water) to compare to the GPS gives me an idea of how much I would want to adjust the speed control to counteract the current.
This is a nonissue in a slalom course since speed over ground, which GPS measures, is all we care about. But in a slalom course, I'm relying on speed control and not paying much attention to my airguide.
Not bagging on your idea. I actually think it's really great. I'm just pointing out one of the advantages, for some, of having a traditional speedo in addition to a GPS speedo.
Previous: 1993 Prostar 205
Red 1998 Ski Nautique, PCM GT40, 310 hp, , Acme 4 blade, Perfect Pass SG/Zbox.
Be kind. Have fun.