Mate Fin Hunter Sportfishing www.moreheadcitysportfishingcharters.com
I flush mine with new water and a cap full of bleach about 2x a year and just top off every trip. When it starts to get cold I add a bit of antifreeze to it. Then flush all out in the spring and add capful of bleach and flush till I cannot smell bleach anymore.
I install water to almost every boat I rebuild. (This is 2008!) some concentrated lime or lemon juice works great also to clean the tank out. a cap full of bleach also works well. you can always dump your water for a long run home if you don't use it... remember water is 8 lbs per gallon. water is great to have onboard.
I keep water in my tanks all the time, mainly so I don't have to remember to fill them before I head out. If you have older tanks or you don't know their history, a shot of chlorine will kill any bugs, then after that the best thing you can do is use the water often and refill the tanks often. If I go a little while without fishing I empty the tanks and refill them with fresh chlorinated water. Unless you have a big contamination problem, the chlorine coming on with the new water each time will keep the water and tanks clean.
Now taste is a whole different ballgame. I don't drink water out of the tanks unless it is run through the coffee machine first. That gives it just the right flavor!