I know on some of the cars and trucks that you can get into the service mode by disconnecting a single brn/blk wire. Then the base timing can be set. If you try to do it without going into a field service mode the pcm will compensate and then it will be way out.
BTW if you have never touched the timing it should not have moved. Therefore no adjustment is needed. There might be a few degrees in the timing chain but not enough to worry about.
I have not played with a marine application pcm so I do not know how they are scanned. Is there a simple code reader or are you going to use a mastertech, tech 2, or a snap on scanner?