Can you post pictures?
On that engine, the oik filter and fuel filter are usually installed on opposite sides. So, I'd guess the smaller filter is the fuel filter.
Fuel lines around the filter are often pencil thickness and metal vs the thumb thickness and rubber lines more often for oil in marine engines, due to the need for a sea water heat-exchanger cooler. Maybe this helps confirm?
The primary oil dipstick is likely the one on top. Some owners and builders add short one lower, right in the pan. The reason is, in marine engines, it is often hard to check the top one while crawling around the engine room. So, the top one is good when the floor is out and access is from the top, the bottom one is good when one is doing the belly crawl with the floor in place.
One question would be, do you see oil on each?
Another would be, is it of the same color and character?
And, a third would be, do the levels correlate, for example, before and after a top off?
Having said the above -- I know nothing about Mainships.