I've heard about people cutting a deep "core" just large enough to fit the neck of a bottle of vodka into, then let the watermellon slowly absorb the vodka from the upturned bottle.

The way I've done it is to cut mellon balls from the watermellon, cantalope, and honeydew. Add fresh pineapple and let sit to blend juices. For spiking I still use vodka, but only a cup or two. My Mom used to add brandy or an orange flavored liquer to the mix.

A plus to spiking the mellon balls and then placing them back into the watermellon hull is that you can also reserve some of the fruit and serve it "non"spiked for those who do not wish to consume alcohol.

The way I learned was to cut a hole in the side of the melon, then stick the handle of a wooden spoon into that hole and mush it around a bit to break up some of the pulp, then put the neck of the bottle of the liquor of your choice into the hole. The amount of booze, depends on the size of the melon.
A nice combo is about a cup of triple sec and a pint of vodka for a small melon.
Punch holes in the rind of the chilled melon, insert straw and enjoy.
Linda C

The triple sec is a good idea. But when spiking a melon, you gotta remember that it is already saturated with water. The best way to spike a whole melon (and I've tried & experienced many!) is to cut it in half the long way, and leave it out in the sun for a few hours. This will dry it out a bit so it can actually absorb the vodka or grain (and triple sec?). Then either poke many holes in each half to the bottom rind using a metal skewer and slowly pour liquor onto it and let it absorb, or (better yet) use a large syringe and inject the melon everywhere. The (important!) refrigerate at least overnight, which will really mellow out the flavor.

Spiking the melon balls separately also works well. Again, it works better if you let them sit out and dry up a bit, then add liquor and let them soak, followed by refrigeration.

Hypodermic syringe, hardly leave a trace.

Makes a nice surprise.

Well I don't know about spiking per se, but we would cut a watermelon in half, use a melon baller to scoop out all the watermelon into balls, then fill up the empty rind with champagne and the watermelon balls - always a big hit with our friends at parties.

I found this "How-to" posting to be very informative. A step-by-step explanation and it worked like a charm for me when i tried it. http://www.tailgatingideas.com/how-to-spike-a-watermelon/

Here is a link that might be useful: How to Spike a Watermelon