I am aware different levels of GPS accuracy. Normal GPS accuracy is quoted at the 50% confidence level. That means if you use the number displayed on your GPS, you will be within that half the time.
Systems that get inches of accuracy are using external data either from another radio or off the web. I am not aware of any marine systems that would be more accurate than a WAAS GPS.
My measurements, which are more or less worst case error in an hour, show WAAS level GPS error between two measurements (like a boat and a starting line that was pinged) to be about 30 feet or 10 meters. A phone GPS would be about twice that.
But the op wants speed and that cannot be augmented except by averaging several runs which is not relevant here. Speed seems to be within a few tenths of a knot.
If anyone knows of systems for marine use that would be more accurate I would love to know about it.