However, that 500m for surface is in ideal conditions. i.e. glass smooth water. Something you don't find or want with a sailboat.
2.4 simply does not penetrate water. Period! So in a boat the Rx needs to be as high in the boat as possible with the antenna sticking up from there. When the boat heels, almost any wave action can block the Rx antenna and hence, your signal. Stuck too deep in the hull and just the water will block it. Most if not all the time.