what can i add to make my water melon drink spicey to serve my friends at a convention

Ginger!

If you have access to Aleppo Pepper, it would be wonderful on the rim of the glass. Rub rim of glass with lime and then roll in an Aleppo pepper/salt mixture.

a bit of beet horseradish

I really like using dried aji amarillo chile in watermelon salad, so I think it would be good in your drink also. It's a very fruity, fairly hot pepper. More about it here: http://www.seriouseats.com/2010/06/spice-hunting-aji-amarillo-chile.html

If you want to add spice but not really much chile flavor, you could always add just some ground cayenne pepper. It's hot enough that you could make your drink spicy before you'd really start to taste the rest of the chile flavor.

If you're in a place that needs this drink now, I'm jealous. Agree with Daniel on peppers. I sometimes use one tiny hot red one for half a watermelon. Also cilantro (buzzed up abd/or garnish). Assume you're adding vodka or gin, yes? Vodka better if you're sourcing it up (the herbals in gin would be drowned out). Rum or bourbon might work well, pplaying off the sweet melon.

Jalapeños go well with melons. I'd roast them first to mellow them a bit before blending them in. Or you can always pickle them.