Fight water in the fuel
any way you can. Keep the tank topped up to discourage condensation
. Replace the rubber O-rings on the deck
fill cap every year, and add a bit of silicone grease to them. Install a Racor
filter if you don't have one. Avoid fuel
docks that have a bad rep. But, in spite of all your precautions, water still may get into the tank.
Here's an idea. Buy one of those rubber squeeze bulbs with the hoses on each side that WM sells for outboard
engines. They come without any fittings on the ends. Cut the rubber hoses to convenient lengths. On the suction side, install a length of 3/8" copper tubing long enough to reach the bottom of your diesel
tank. Secure it with a hose clamp. After the boat has sat quietly for a time, open up the tank's access port, or remove the pickup tube if you don't have one, and gently insert the copper tube into the tank's nether regions. Squeeze the bulb and extract a pint or so of diesel into a clear plastic peanut butter jar (cleaned up, of course).
You can then judge the general cleanliness of your fuel, and also remove any water or sludge that's accumulated in the bottom of the tank.