A cruise just isn't a cruise for me without my evening martini or two...and this is a real martini made with gin. I've never accepted the bastardization of martinis with vodka, chocolate liquer, etc.
Although ships have distilling plants they don't totally rely on them. When they can they will fill their tanks with potable water from a pier and this is also filtered and tested. This is what we did in the Navy too - we'd buy water from a port whenever we could since it simply is a faster way to top off your potable water tanks.