Martin,
You might check to see if it can be inserted from within the boat, if the threads are there and you have room to turn a wrench, the plug doesn't really care. Also, you might put some anti-seize lubricant on it liberally, that's a greyish goop you can get at most auto parts stores. You can tighten the plug with no danger of its being "welded" in place by corrosion. And..the stuff is really good for your trailer lug nuts, they too will not "weld in place". Thanks to Bob Burks of SeaPal and owner of a tire store for that info. We keep a spare push-in expandable plug on a velcro leash right by the plug hole so we have one available in case of emergency.