It shouldn't be necesary more than twice a year
That is, IF you used the recommended amount of bleach, and IF you followed the directions to make sure the solution gets into the plumbing and stays there for the full time, as well as in the tank. I'm in GA, which has a lot longer hotter summer, and where the boat sits in much warmer water, than NY...and I rarely had to recommission my fresh water system more than once every two years. You're right that you should not add a small dose of bleach to each fill. Some questions: how often do you use your boat? Do you have a dock water connection, or do you use your tank water? How often do you fill your water tank? When you fill the tank, do you stick the hose in the fill immediately after turning on the water, or do you let the water run till all the water that's in your hose and the dock supply line has been flushed out before you start filling the tank? Do you fill from a well, or from a "city" water main? Water that just sits is gonna turn stale--a problem for boats with pressurized dock water connections. The plumbing in boats that sit provides the idea warm, dark, damp environment for "critters" to thrive. Keep the water flowing through the system...turn over the water in your tank(s) as often as possible. If your boat plumbing provides an idea environment for "critters," the dock supply line and your own hose provide an even better one, because they sit in the nice warm sun. If you don't let the water run till all the water that's been sitting in 'em has been flushed out, you'll just transfer those critters to your boat's fresh water system. If you fill from a well, the water hasn't been treated...so it's more susceptable to "critter" growth than city water. Only if that's the case should you add ANYthing to the tank with each fill..about a capful of bleach to a tankful. More, and your water will taste and smell like an over-chlorinated swimming pool. And if all else fails, the answer may be a filter on the faucets.
