If you store your boat on the hard- every spring I stick a copper tube at the bottom of the tank ( thru the fill port- bend it do it goes deep)and use a hand pump to suck out a sample-clear glass gin or whisky bottles work fine. You may want to do several extractions. You're looking for water and dirt in the bottle. For gasoline tanks, you can decant the fuel thru a coffee filter in a funnel back into the tank. Pitch the last inch or so of grunge in the bottle, and marvel at the crud your fuel filter doesn't have to eat. After sitting for several months in the yard, any crud that's going to settle to the bottom has done so. Yearly extractions will tell you whether you're keeping up. Other posts are accurate- keep extra filters on hand.