Step back from the problem.
If you are proposing to lug umpteen 5L cans or a jerry can out to he boats to refuel, why not just buy a couple more 20L fuel tanks, and carry them out & swap them over? If you put a connector on the tank rather than a fixed hose, you also remove a potential trip / damage hazard to the fuel tightness of the portable tank.
That way you remove a lot of risk with the "offshore" refuelling, and only need to worry about the storage & getting the fuel out the pumps & into the tanks side of the risk assessement. It also means you are alot less likely to contaminate the fuel, and will give you a lot more operational flexibility.
Risk assessement stuff above still applies!