Fuel (all hydrocarbon fuels - even LPG) contain water in differing amounts. I dunno what it is for Diesel, but for Aviation Kerosene the limit is 30ppm. The refiners don't worry about it too much because, in small quantities water can actually aid combustion - it's when there is too much it can be a problem. Two things...
1. Entrained water doesn't mix with oil and so settles out over time (several hours), causing the "condensation" you see in the bottom of the tank. ie. The water has "condensed" out of the fuel.
2. Most hydrocarbon fuels significantly change in volume with changes in temperature, so if you buy fuel at, say 70degF and fill your tank on a 60degF day, the fuel volume will decrease gradually drawing a small amount of air in through the vent. Any moisture in the air will then settle on the tank walls and add to the "condensation" at the bottom of the tank.
The way to avoid issues with water in your fuel?? You have two options: Either pump it out before you go or recirculate it though your fuel system on a regular basis..
Yes or .81oz EVERY DAY THE CONDITIONS ARE RIGHT FOR CONDENSATION!!!!!!!! Someone remind him there are 365 days per year and tanks in boats should be expected to last well into the 30 year mark..?? Even if your tank could only hold .01 oz of water vapor over time it ADDS UP!!!!!
Halekai, there's a small problem with your premise - an empty tank doesn't have any fuel in it.. If what you're saying is correct, over time an empty tank would magically fill itself with water - in practise it doesn't.