If by 12V plug you mean the typical cigarette lighter outlet, they are supposed to be fairly standard, but I've seen some cases of incompatibilities over the years.
My guess is that either your boat or the air pump has a slightly odd socket/plug to make the two of them not work.
One cheap solution might be to go to Wal-Mart or wherever and find one of those multi-outlet adapters that turn one outlet into 3 or 4. You might get lucky and find the outlets on that work better with your inflator.
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