Diesel fuel isn’t much different from jet fuel, glorified kerosene. We used to be required to add an anti bacterial, (trade name Prist) to our jet fuel. Those “oily” type fuels will grow anaerobic bacteria, they live off the paraffins in the fuel, creating a black sludge.
if the boat sat for some time in a tropical climate, that, & water contamination are suspect.
its not enough to just replace a filter or water separator, if the tank is fouled, more contaminants will just result in repeating the problem.
For an offshore boat, I’d have a qualified marine shop go through the fuel system from stem to stern.
Draw a fuel sample & have it checked/tested. You can also get a water indicating paste. We would smear some on the end of a dipstick, push it to the bottom of the bizjet tanks. It would instantly show the presence of water.
