On my '86 3600 the drain plus is on the bottom of the boat in the back, not on the transom. I have to remove my fish box to get at it. I never remove it. When I haul out, I just use a wet/dry shop vac to remove any left over water.
I had the same boat a few years back, if I remember correctly it is located between the engines. It is a T shaped plug to drain the bilge. There is no plug in the transom.