My onboard fuel polisher has a small electric pump
and is built around a Raycor 900 filter. I have installed two little brass valves, one on the intake hose and one on the output. This allows me to divert suction from either the main fuel tank
or an auxiliary intake hose, and to divert output to either the main fuel tank or an auxiliary output hose.
To transfer fuel from any of my six 5-gallon plastic jerry jugs, I simply take the auxiliary intake hose and insert it into the bottom of the jug, setting the valve to use that hose, and leave the output valve in position to transfer directly to the main fuel tank.
It takes about 5 minutes to transfer most of the fuel in a jug to the main fuel tank, after which I can either pour the remainder into the fuel opening atop the tank if it's calm enough or wait to do it later.
With this system I can add the contents of three jugs (15 gals) in less than 30 minutes, including setup and cleanup.
The system works for me in both calm conditions and in pretty rough seas.
BTW, the fuel goes thru the Racor
900 filter on its way to the fuel tank.