The issue of water uptake from the atmosphere is simple, its called chemical equilibrium.
When diesel oil is processed the heat generated essentially dehydrates it and drives off all water. All tanks used to store it are vented to the atmosphere and the water vapor in the atmosphere eventually 'equilibrates' into the oil and eventually 'saturates' it to the maximum degree possible and according to the temperature of the oil being stored. A tank containing oil and vented to the atmosphere will 'lag' its changing temperature with the ambient atmosphere as the 'outside' temperature swings up and down; thus, will have continually different water saturation equilibrium and that equilibrium drive will eventually bring the oil to it maximum saturation with water .... until the temperature cools and some water separates, and drops to the bottom, the tank contents heats then cools and more water (as vapor) enters the oil to reestablish 'equilibrium' in the oil.
Empty tanks do not magically fill with water, do they? Yet partly filled or full oil tanks do accumulate water in their bottoms.
For 35 years, as a filtration engineer including deep involvement with ultra-pure chemicals, super high tech dielectric oils used in electrical transformers, etc. and if any container these are stored has any 'communication' to the wet atmosphere (vent), water will migrate into these chemicals and eventually saturate it ... sometime even through 'hermetically sealed' containers. Its just simple chemical equilibrium and ultimately the oil will saturate and with temperature swings and lags of the contents, water will separate fall to the bottom by gravity ... the tank and it contents now behaving like a chemical pump, continually extracting water vapor from the atmosphere and allowing to accumulate at the tank bottom.
In the oil industry, when oils are required to be nearly 'water free', we will constantly purge the tanks with bone-dry low pressure air or nitrogen or chemical foams containing nitrogen, etc. Even those million gallon storage tanks vented to the atmosphere will eventually become saturated with water if such steps to prevent migration of water from the atmosphere isnt taken.
Why I recommended the Racor 'water shedding' membrane systems is that it removes the 'free' or 'visible' water from the oil and you will then not 'slug' a hot fuel injector with relatively cold water thus potentially breaking the injector tips.
;-)