We do our sailing in Florida and the Bahamas (Exumas). The water didn't always taste very good, and would sometimes deteriorate to a noticable degree before we used it, particularly the deck jerrican load.
Like some of the other posts, I decided we couldn"t really do better than a whole house filter from Home Depot or similar. I installed it below the sink, downstream from the
pump, upstream from the faucets, with a plain charcoal filter. We also dose the water supply with small amounts of clorox bleach to discourage microorganisms, green weed etc. (appx. 2 tbsp/23 gal.) This leaves a faint hypochlorite taste/smell, but the filter removes this. We've had good water this way for many seasons. Only thing to watch for (down south anyway) is a grey webby-looking mold that may develop on the outside of the filter. Easily seen - if you don't catch it visually, your first indication may be that the water doesn't taste so great anymore. We go through two filters in about five months cruising down south. Up north they might last longer - but they're cheap. Keep a spare on board.
Michael Flood
"Mole End"
Bayfield 29