I have bad gas!

I would start by checking for a drain plug, then like you said 5-10 gallons of clean fuel maybe 91-93 octane and marine grade sta-bil. Before cranking it over pull the fuel line at the fuel rails or carburetor and change out any fuel filters you have. crank it till good fuel gets to the rail or carb then reattach the lines. flush the old gas from the area with a hose then attempt to start it up. if you have water in your tank you may have rust in your tank and it will plug up your fuel filters, injectors and carb jets you may have to change filters a couple of times in the process so don't get discouraged. you may get lucky and not have any or much water as well but I wouldn't neglect to flush the lines; plugged injectors or carb jets is a pain especially when it can be avoided.