Roo said..decrepit said..
apppro and GPSTC approved both give our divisions, I think waterspeed is more general.
Waterspeed has all the GPSTC divisions as well as quite a few more that are on GPSSS. Probably best to investigate further rather than making uniformed decisions based on what you think. Accuracy is based on the watches GPS capabilities rather than the app, all the apps use the same data from the watch. Even the data coming from the last few generations of Android phones has a high level of accuracy and gives error estimates.
Telling grandma how to suck eggs.
Personally, I have found GPSTC to give excellent on-the-water feedback, and it is compatible with GPSTC and GPSSS. Installation is fiddly initially, but works fine with the provided detailed instructions.
I have found that the Motion and Garmin 255 are almost always within 0.2 knots, varying either way.