I plan to put one of these in my boat, too. My 52 gallon water system simply isn't enough to keep my family happy the way we use our boat as a floating condo on weekends. The Mrs. and Junior think that shower is going to run forever!!!
I do use the water tank, though, so my thoughts were to put the city water system in, but only use it when we're on the boat as an alternative to the water tank. Plus, I would think you should use up the water out of your tank every few weeks so it doesn't get stale. That seems to be a reasonable compromise. I hear ya on the safety issue - I saved a friend's boat when I went into the cabin to "steal" some sugar for my coffee and stepped into a foot of water at the bottom of the ladder. It too had sprung a leak in the city water system...
PS - on older boats at least, as some have said already, be sure to put a check valve in to protect the fresh water pump. They are very cheap to purchase...