Rinse the outer rind of the watermelon thoroughly, and pat dry. Set the watermelon in a position so it will not roll over. Press the tip of a funnel through the rind of the melon. If using a plastic funnel, you may need to cut a hole.
Situate the melon on a towel in the bottom of the refrigerator or on the counter. Pour rum into the funnel a little at a time, refilling as it seeps into the melon. I start the afternoon before, since we usually leave to go to events in the morning. Allow the melon to marinate at least a few hours, before removing the funnel. Slice just before serving.