You said this is only showing up with a hot engine and that when you ran it on muffs you did not get it up to a high temp? If so, take the boat out on the water wit the mechanical gauge attached and see what the gauge reads once you get to the higher temperatures where the problem was occurring. If it doesn’t react the same way as before, then try the wire change. On mine, the dash gauge has a telltale or light and audible alarm that are connected in line. If the dash gauge read low, the telltale light and alarm sound. As a backup to this system, the 5.7 GSI has another oil pressure switch that is connected to the ECM. If this switch detects an issue with oil pressure it will put the engine in limp mode. Mine never went into limp mode so I was confident it was not really a pressure issue. The mechanical gauge confirmed that and I then went through troubelshooting the dash gauge/oil sender/wiring.