I have a diesel pick-up truck and someone advised when I got my first diesel to always keep the fuel tank as full as possible, especially in the winter months, even if using the vehicle all winter long. I asked why? They said, a full tank will reduce the possibility of water condensing on the inside of the fuel tank, and the very last thing you want happening to your engine is to have water in your diesel fuel.
Well, I certainly never understood how a tank with a closed cap can attract water vapor, if it didn't exist inside the tank to begin with. But, being much greener back then and he was one of those old timers that had been around the barn a few times, I always took his advise and never had water problems inside my diesel fuel tank. I keep mine full all the time. Ten gallons low and I fill up. Sitting in the garage for any length of time (like 2 or 3 days planned no driving), always filled full.
Needless to say, I've never experienced water in my fuel. So, I don't know if the old man was on-to-something, or full of BS. But, I've never had watery diesel fuel. So, who knows?