Ayuh,...... Welcome Aboard,...... Tell us more about what, Exactly yer motor is,....??....
Year,..??
Carbed, or efi,..??
Serial number might help,.....
Oil there could be comin' from the intake gasket, though unlikely,....
It could also be the valve cover gasket on that side,.....
Or, it could be signs of blow-by from the crankcase vent system,.....
Pictures would no doubt help, say from 6", 'n 12", 'n 2',.....
The best way to identify the leak is by close inspection,......
A couple of hints, oil flows downhill, just like water, the leak won't be below the stain,.....
The cleanest oil you see(no dust or dirt, not that there is much of either in a boat's motorbox), is gettin' close to the leak, look higher,.....
If it's blow-by from the vents, a compression, 'n leak-down test should point ya in the Right direction,.....
Cleanin' the area, real Clean, 'n runnin' it under load is Best,....
Though that's impracticable in Detroit this time of year,....