there are many ways to prevent water uptake into oil, many ways to affect dehydration of the oil once saturated.

The conservator as noted in another post ... can be enhanced with an occasional recycle of the conservator oil through water absorption starches (after vacuum flashing) down to relative few ppm(w) water, etc. ... the conservator isnt 'perfect' and still allows retrograde saturation over a 'long' term.
The use of nitrogen filled 'foams' as blanketing agents, etc. etc. etc. The simple padding of the tanks with dry gases .... but always a risk due to equilibrium sending moisture retrgrade againt the 'dry' flow along the pipe walls.
The most common way used to 'reclaim' / restore dielectic oils to High Kv in the power gen. industry still remains vacuum flashing and then running the transformer and/or tap changer oil through a cellulosic medium with a bound water absorbing starch (hydroxymethylcellulose ... the stuff in baby diapers).

FWIW - Ive long used a simple silica gel adsorption bed (chamber) on my tank vent that includes 'signal' gel that changes color when the gel needs to be regenerated ... and simply change out the gel when I see the saturation front (color change) close to the tank side of the chamber.

The simple and elegant fact remains that oil, fuel oil in the case of boats, will uptake water to its ambient level of saturation equilibrium conditions if exposed to the atmosphere.
The simple key here for a boat owner, is to have the least necessary amount for oil in the tank, the least amount of oil mass results in the least amount of water uptake - simple 'chemistry'. The least amount of water (in all its various phases in oil) results in the least amount of microbiological growth and the least amount of 'resins' (products of micro-biotic metabolism) from depositing on the tank walls, etc.

I say again, drain the tank for long term inactivity and storage, especially if one's tank has a history of becoming contaminated with the typical fungals that easily clog 'filters'. Trucks and other vehicles that constantly refill their tanks dont have such a problem as do boats that sit around for weeks and months with 'stagnant' tank oil thats vented to the atmosphere.