Better to prevent the entrainment of water for long term effectiveness; barring that, treat and pump out any tank bottoms.
National Chemsearch used to sell a product that would allow for rapid separation after agitation, such as what happens when fresh fuel is dropped in a partial tank, it also contained a basic biocide.
Don’t remember the name or whether they still sell it.
I once was in charge of 19 gensets for a VA hospital with a combined 7.5MW capacity and ordered and controlled all the fuel being used.
We eventually replaced the in-ground tanks and upgraded the system.
On ships seawater seems to get into everthing plus the tank cooling from the sea temp and high humidity create plently of water in the fuel tanks that is well agitated. as fuel oil and water king it was my job to get clean fuel to the propulsion and auxiliary plants. Land based systems are much easier to deal with. I wrote a short story once about a genetically created bug that ate garbage and excreated oil. Worked great until it got out of the tank and ate Brooklyn.