Either fuel hose that is rated for Diesel
that meets Coast Guard standards or refrigeration
grade copper tubing with flared compression
fittings. The first is easier and the second is more fire resistant, never rots and is what is required for inspected passenger vessels.
You might also want to consider a remote
shutoff for your fuel supply and return valves. This is a stainless steel
wire pull that turns off the fuel supply and return valves at the fuel tank. This allows you to shut down the supply of fuel to the fire remotely, such as from the boats steering
station. Many boats have burned to the waterline because there was no way to get to the fuel shut down valves because the fire was at those same valves. Remote
fuel shutdowns are required on USCG inspected passenger vessels.