For large watermelons I've used an entire fifth by cutting a hole big enough for the neck to fit in allowing it to sit there a good 6-12 hours and even longer (24 is good). Make sure
you keep the melon cold in a cooler or fridge until serving time. I prefer rum over vodka and also prefer lower proofed alcohols like Malibu because watermelon meat is denser than other fruits. Also, chill the bottle beforehand so that the warm alcohol doesn't eat away at the dense meat inside. When ready, set it out at your party so that everyone can see how cool it looks. When it's time, slice it up and enjoy!

I have also mixed alcohol together like half vodka and half Midori in the bottle. Go ahead, experiment!

At your next pool party you can also cut a hole in a watermelon and stick a bent coat hanger (or something like it) inside to gut it out. Then fill it with your favorite chilled party beverage, plug it and throw it into the pool. It will float around. Whenever you want a drink you pull the plug, chug, then push the plug back in.

You can't use the same plug that you cut from that melon. It must be cut from another melon because it needs to be a slight bit bigger.

You can make drunken olives by getting a big jar of olives. Drain out 3/4's of the olive juice then pour in a mixture of gin, vodka, and dry vermouth. Let it set overnight in the fridge.

*The jars are cheaper at a Sam's Club or the like.