Ok; got shot down again. All I can say is it worked for me; others have different ways. The OP asked a question; I gave a answer. I thought a Forum was of differing opinions; maybe that is why they killed JC.
Um...guys, he was talking about using the shop vac
"backwards"
I.E. turning the vac "around" and allowing it to provide pressure to the tank. There's essentially little chance you could get a spark that would have to travel all the way down the hose and into the tank AND there would be NO fuel vapor traveling through the vac hose or vac.
Now having said that, you would have to have the VAC not only outside the boat, but I would want a long enough hose so I could put the vac AT LEAST 50 ft away from the boat!!
(I STILL WOULD NOT DO IT, EVER!!)
ALL brush type electric motors (AC and DC) produce a "SHOWER" OF SPARKS!!
Even using a battery powered electric drill will produce a a spark that can ignite fuel vapor. They are UNSAFE when used in an ignitable atmosphere. (unless they're certified explosion proof https://www.dietzelectric.com/pdfs/hazard-loc-class.pdf )
The quickest and easiest way to drain a tank is to siphon the fuel out though the fuel gage sender hole with a large diameter hose.
Raise the bow up
Go to home depot and buy 20 or more ft of 3/4" clear vinyl hose, some plastic PVC pipe and a brass ball valve.
Put a piece of pipe on the end of the hose you'll put in the tank and the ball valve on the end that will be at the fuel receptacle end.
Fill the hose with fuel first. DON'T use your mouth!!........Just fill the hose with a small funnel. Have a helper hold the end with the valve (open) while you fill from the other (tank) end. (hold the valve end at the same or slightly higher level as the "fill" end)
When the hose is full, close the valve, place the tank end (with the pvc pipe) in the tank. Raise the valve end (with it open) high enough up to purge air out of the hose and then close the valve.
Then get your containers ready at the valve end (on the ground)
Have your helper open the valve and fill each container........ closing the valve between each fill. You use the end in the tank with the PVC pipe on it to move around and "vacuum" any debris and/or water out.
With the bow up, you'll get nearly all the fuel, water, and debris.
DO THIS OUTSIDE AWAY FROM YOUR HOUSE OR ANY OTHER IGNITION SOURCE
Have a fire extinguisher handy (preferably HALON or equiv)
I would use 2 year old gasoline in my car mixed 1:5 with fresh gasoline.
Throw the Seafoam, Stabil, or other Snake Oils away. They should be used PRIOR to gasoline storage.......They won't do anything for old gasoline.
Cheers,
Rick