Diesel bug is a slime of bacteria and fungi that lives at the boundary between water and fuel in a tank. On a boat it gets everything it needs: fuel that sits for months, condensation, warmth and often a leaky deck filler. It grows on the tank bottom until rough water stirs it up, and then it blocks the filter and stops the engine.
Ben Sutcliffe-Davies, a marine surveyor writing in Yachting Monthly, says the organisms like 5 to 70°C, thrive around 30°C, and come in over 100 identified variants.
What a surveyor checks
Look into the primary filter's bowl before the engine runs. Clear fuel is good. Water at the bottom, dark sediment or a brown jelly are the signs. Draw a sample from the tank drain or filter into a clear jar and let it settle. Check the deck filler cap and seal: Sutcliffe-Davies often finds caps that let water down the filler pipe.
In the report
"Primary fuel filter bowl: water layer and dark sediment, brown deposits on the element. Tank sample shows the same. Have the tank inspected and cleaned, polish the fuel, renew filters and the deck filler seal." See diesel bug.