Buy the Mercury/Quicksilver Fuel/Water separators from a Mercury Dealer.
As for oil changes, also purchase the Mercury kit to add to the bottom of your oil pans, that have a hose permanently attached to the drain hole. Extend the hose out your bilge plugs and drain this way. MUCH better that slurping it out the dipstick; as this leaves roughly a quart of dirty oil in the pan each time you change the oil. Yuck!
http://www.mercruiserparts.com/ is an OSO sponsor and member, and can get you everything you need PDQ.