It gets a little slow around here this time of year but I am certain that others will chime in next week.
First of all, which make and model diesel is it? Some here will have the exact same engine and be better able to answer your questions.
I would start out by cleaning up all the oil in the bilge and the bottom of the engine. Drain out as much of the old oil from the engine as you can while you are at it - you do not know when it was last changed anyway. They make oil absorbent pads for this but if there are no EPA regulations where you are then you can use anything but the pads work best as they do not absorb water. Oil change pumps like this work great: http://www.defender.com/product.jsp?path=-1|311|296536|314179&id=220125
Oil pads like these will be useful, get a bunch: http://www.defender.com/product.jsp?path=-1|10918|309314|311479&id=106252
Once you have drained out and cleaned up the old oil I would line the bottom of the engine with some new oil absorbent pads. Next I would make sure that any oil drain plug that might be under the pan is properly in place. Next add a few quarts of fresh oil and see if you can see or feel any drips under there. If no drips then fill oil as per dip stick level. If it is dripping you will see it on the white oil pads which will give you an indication of where it is dripping from (not necessarily where the leak is). Get a small mirror and a flashlight to be able to inspect the bottom of the engine.
Take pictures and post them here. Others will be along to chime in. There are probably a few most likely suspects that could cause an oil leak: oil pump, filter, lines, water pump seals or even a blown gasket somewhere. The procedure I outlined above should at least help you start to track down if and where you have a problem.
If it is a slow leak it may not be that big of a deal as many older engines burn or lose a little oil here and there. You just do not want to have a gushing oil leak such that there is a slick on the water around your boat. In US waters you can be fined for this.
"The cure for anything is salt water~ sweat, tears, or the sea." ~Isak Denesen
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