To be honest, you are both (@Tom Bishop and @DuncanDoenitz) engaged in something of a sidebar debate. For what it's worth, ASIs in aircraft aren't 'unreliable', they are both reliable and extremely important. They are also extremely useful for navigation, but they have limitations. As Tom rightly points out, they don't measure groundspeed, or 'gs'. Traditional navigation would involve correcting for the forecast wind, using either mental reckoning or the more sophisticated Dalton computer, and then updating position using either visual features or a ground based beacon such as VOR/DME. GPS has clearly made that process a lot easier, but then presumably FET proponents would take issue with the concept of a constellation of geo-stationary satellites.
For ships at sea a similar station exists. Sailors can measure their speed through the water, but cannot account for the movement of the water without measuring by some external means, such as celestial sightings (deeply problematic from a FET perspective) or again, GPS.
But leaving all of this aside, there is a much more fundamental truth that we need to establish here. Could a FET proponent, maybe @Tom Bishop, for example, please tell us how far the race route would actually be on a FET map, and therefore how fast, in terms of groundspeed, the boats would have to travel in order to make the kind of times we are discussing. We can then see whether or not the speed of water movement required would be credible.
I would also like @Tom to explain how the boats know where they are - how does GPS work, for example? Does celestial navigation work?