Tom,
if its your bilge drain, Bob is right, be careful with heat because the plug housing is simply screwed into the transom with self tappers and a bit of 5200...too much heat may break the seal of the adhesive and let water in...if you have to use it and get it hot, I would then remove the plug housing and reseal it again.
If you are talking about the G drive, it should be no big deal to use some heat...its just the brass plug and aluminum G drive that have basically welded together from some corrosion.....use some heat to make the plug expand in the hole, spray some WD40 or similar right at the threads and then walk away and let it cool 20 minutes....the heat will expand the interference fit and break that "corrosion weld" allowing you to break it free. Its best to use the tightest fitting wrench possible, that contacts all or most sides of the plug to keep it from stripping...either a good fitting open end wrench that will grab 3 full sides, or a 6 point socket, not 12 point...I prefer the open end wrench and a pipe extension for leverage....this should give you enough ooomph to break it free....if its completely rounded off...you will have to drill out the brass plug which is soft and not that bad...