Pre-launch: Check all your through-hulls and seacocks for signs of corrosion or other failure, lubricate and re-bed as necessary. Check all below-waterline hose connections and replace any cracked or broken fittings or corroded hose clamps. (And don't limit this exercise to your sanitation system!) If you haven’t done it in at least two years, change every impeller—in the electric toilet, the macerator, the engine intakes--on the boat, even if it looks fine. Each time an impeller pump starts up, it runs dry at least briefly which wears down the edges of the vanes. Unless it’s run dry long enough to really fry it, you can’t see the wear, and there’s only about the width of a human hair between an impeller that still fits tightly enough in the housing to pump water and one that doesn’t. If you have access to water, now's a great time to really CLEAN your bilge, sumps and chain locker--while the boat's out of the water and can really dry out. You won't smell them now, but there are a lot of odor-causing “critters” growing in all those places, just waiting to multiply and STINK when the weather get hot If you haven't rebuilt your toilet in at least 5 years, now's the time to do it--while it's clean and dry. Preventive maintenance--new seals, gaskets, valves, etc--reduce the odds of having to make repairs by 99%. Whether you rebuild or not, lubricate your manual toilet: open the pump and give it a liberal slathering with SuperLube—the same thick teflon grease that was in it when it left the factory. Not only will it keep your toilet pumping smoothly for a full season, but it will protect the seals and pump housing from grit and dirt. Check all vent lines--on fuel, water and holding tanks--for obstructions and clean them out if necessary. Pressure test your holding tank for leaks. Check hoses for odor permeation and replace if necessary--it's a much more pleasant job in cool weather and when the system is clean and dry--than in hot when they're full of sewage. Replace check valves in vented loops. Post launch: Test your toilet...make sure it's working correctly. Especially in electric toilets, things have a way dying over the winter. The time to find out is BEFORE you stock the cooler and take the kids out for the first spring cruise! Rinse ALL the antifreeze out of the system thoroughly with fresh water...but don't add any treatment to the tank until you're ready to use it. Put that down the head with the first "real" flush. Fresh water system (pre or post launch): Although most people think only in terms of the tank, the plumbing is actually the source of most foul water, because the molds, mildew, fungi and bacteria which cause it thrive in damp dark places, not under water. Many people—and even some boat manufacturers—believe that keeping the tanks empty reduce the problem, but an empty water tank only provides another damp dark home for those “critters.” There are all kinds of products sold that claim to keep onboard water fresh, but all that’s really necessary is an annual or in especially warm climates, semi-annual recommissioning of the entire system—tank and plumbing. The following recommendations conform to section 10.8 in the A-1 192 code covering electrical, plumbing, and heating of recreational vehicles. The solution is approved and recommended by competent health officials. It may be used in a new system a used one that has not been used for a period of time, or one that may have been contaminated. Before beginning, turn off hot water heater at the breaker; do not turn it on again until the entire recommissioning is complete. 1. Prepare a chlorine solution using one gallon of water and 1/2 cup (4 oz) Clorox or Purex household bleach (5% sodium Hypochlorite solution ). With tank empty, pour chlorine solution into tank. Use one gallon of solution for each 5 gallons of tank capacity. (The simpler calculation is 1 quart of bleach/50 gal water) 2. Complete filling of tank with fresh water. Open each faucet and drain cock until air has been released and the entire system is filled. Do not turn off the pump; it must remain on to keep the system pressurized and the solution in the lines 3. Allow to stand for at least three hours, but no longer than 24 hours. 4 Drain through every faucet on the boat (and if you haven't done this in a while, it's a good idea to remove any diffusion screens from the faucets, because what's likely to come out will clog them). Fill the tank again with fresh water only, drain again through every faucet on the boat. 5. To remove excess chlorine taste or odor which might remain, prepare a solution of one quart white vinegar to five gallons water and allow this solution to agitate in tank for several days by vehicle motion. 6. Drain tank again through every faucet, and flush the lines again by fill the tank 1/4-1/2 full and again flushing with potable water. People have expressed concern about using this method to recommission aluminum tanks. While bleach (chlorine) IS corrosive, it’s effects are are cumulative. So the effect of an annual or semi-annual "shock treatment" is negligible compared to the cumulative effect of holding chlorinated city water in the tank for years. Nevertheless, it's a good idea to mix the total amount of bleach in a few gallons of water before putting it into either a stainless or aluminum tank. An annual or semi-annual recommissioning according to the above directions is all that should be necessary to keep your water tasting and smelling as good as anything that comes out of any faucet on land. If you need to improve on that, install a water filter. Just remember that a filter is not a substitute for system mainteance, and that filters require regular inspection and cleaning or replacement. (the above is excerpts from the book at the link below)
