Just a thought from one who had a similar oil-skim around the motor in the water. I noted this previously, but am happy to repeat.
On our Alumacraft Classic with 60 Merc. We put in in May for the first time that year. There was an Oil-skim on the surface around the motor. Of coarse it is very upsetting. When we pulled the boat and got back home, we removed the upper engine cover and noted there was a 3/8" deep Pool of oil 360 degrees around the base of the engine casting where the lower covers attach. There was oil residue from the Filter down to to the base also. The oil ran from this pool, down the inside of the lower covers and out into the water.
Seems the Storage Facility (slob) who changed the oil and filter never thought of catching any oil during an obvious S-L-O-W filter removal.
All-in-all, we took towels and cleaned the standing oil out of there. No more oil-skim that year!
Just one option to check.
