I've traveled throughout the world when I was young, and there was no place on the planet, other than the U.S. and Canada where I considered the water purified enough to drink without additional treatment for various forms of bacteria. I've even encountered bad water in several U.S. marinas, locations where the marina is on a well, the water lines
to the dock are very long and very, very old. Consequently, the lines
must be used for quite a while in order to flush them thoroughly and provide relatively clean water.
I keep a couple cases of bottled water on the boat at all times, mainly for brushing my teeth. My water tank is 70 gallons, and gets cleaned regularly with a mix of chlorine bleach and Dawn dish detergent. I leave the solution in the tank for a couple days, which allows the rocking action of the boat to slosh it around like an agitator in a wash machine. I then pump
it through the entire freshwater system before sending it overboard through the drains. Yeah, I know, it's not environmentally friendly, but then again, neither am I.
The next step is thoroughly rinsing the tank and system with a couple hundred gallons of clean, fresh, water, then refilling the tank. When the tank is completely filled, I add a cup of chlorine bleach, pump
water through the lines
until I can smell the bleach, then it's clean enough to wash dishes and vegetables.
I used to routinely test the water for bacteria, but after two years of testing and no bugs I stopped doing that. Besides, it was getting expensive. Somewhere on the Internet there is a formula for how much chlorine bleach to use per gallon when treating water. If I recall, it was about 16 drops per gallon, but I could be wrong on the formula. If you don't smell chlorine after 16 drops, add another 16 drops and wait 30 minutes. If you still don't smell chlorine, discard the water and find another source.
If you're worried about the chlorine bleach causing health problems, believe me, if you contract C-diff from contaminated foods and water it's a lot worse than anything you will experience from the chlorine bleach. Without proper medical treatment, C-diff can kill a healthy person in a matter of a few days.