I'm gonna throw one more out here. The starter is upside down and you are looking at it from above. Make sure you are turning the right direction.
It happens, ask me how I know.
The Starter motor bolts are generally upside down, so it's unlikely that any penetrating oil is going to go up the threads into the threaded area. A longer wrench is usually the only choice (short of removing the engine from the boat).
Gary
everyone tells me that you will be changing starters and alternators regularly, it is just a way of life with Mercruisers GM 350's and inboards.