Water....
There is no good answer to your question. A lot depends on where you are and how the plumbing is cared for. I personally would not drink any water from a marina dock hose without first letting it run for 10 minutes or more and then treating it with Clorox per the link below. The same holds true for the water in your boat: play it safe and put some Clorox in it as it's hard to keep all the nasty cruddy stuff from forming in your tank or water hoses. To me, it's much easier and safer just to bring along jugs of bottled water whenever we cruise....