Rosie, I, too, chased a recipe for pure watermelon wine for some time. I finally obtained one, but after making the wine I discarded it. It simply is an inferior wine by itself. It lacks body and is thin. Further, the watermelon flavor, which is what I expected and wanted, did not survive fermentation very well. The taste was bland and very tired. It tasted similar to a soft drink that has gone flat. No--the soft drink would still taste better. I guarantee you that pure watermelon wine, at least made from that particular recipe, would be a huge disappointment to you.
I have several recipes for excellent "watermelon wine," but each and every one of them requires the addition of another fruit to give the wine body and additional flavor. This is not unusual in winemaking. Many of my favorite wines require adding raisins, figs, dates, or some other fruit to the ferment to save what otherwise would be an unacceptable wine. I hope you will not be too disappointed by this, but instead will rejoice that you have several options available to you for making excellent watermelon wine.
I have already posted a recipe for watermelon-peach wine. I will post one below for watermelon-elderberry wine and another for watermelon-grape wine. Each of these (including the watermelon-peach) uses just enough of the second ingredient to enhance the watermelon flavor without overwhelming it. I hope you will try one of these.
Cut the rind off of melon, cut melon into one-inch cubes, remove loose seeds, and put melon and any free juice in primary (crock, plastic pail, etc.). Grate the yellow thinly off two lemons, then juice the lemons and add the juice and zest (gratings) to primary. Add dried elderberries, pectic enzyme and yeast nutrient. Add water to make up 1 gallon. Stir in sugar and stir well to dissolve. Cover primary with cloth, wait 12 hours and add wine yeast. Cover and ferment 3 days, stirring daily. Strain juice into secondary (demijohn) and fit airlock. Ferment 30 days and rack, topping up with water into which 1/3 cup sugar has been disolved. Add one crushed Campden tablet, refit airlock, and rack every 30 days for 6 months. Stabilize (1/4 tsp potassium sorbate and another crushed Campden tablet) about a week before bottling. Allow to age at least 6 months in bottles, but improves with additional age. [Author's recipe]
Cut the rind off of melon, cut melon into one-inch cubes, remove loose seeds, and put melon and any free juice in primary (crock, plastic pail, etc.). Thinly grate the yellow off two lemons, juice the lemons, and add the juice and zest (gratings) to primary. Separately, wash, destem, and crush the grapes well in a bowl. Add grapes and grape juice and crushed Campden tablet to primary. Add water to make up 1 gallon. Add sugar and stir well to dissolve. Cover primary with cloth, wait 12 hours and pectic enzyme and yeast nutrient. After additional 12 hours add wine yeast. Cover and ferment 5 days, stirring daily. Strain juice into secondary (demijohn) and fit airlock. Ferment 30 days and rack, top up, refit airlock, and repeat 30 days later. After additional 60 days, rack, top up, and stabilize (add 1/4 tsp potassium sorbate and another crushed Campden tablet). Wait 10 days, sweeten to taste and bottle. Allow to age in bottles one year. [Author's recipe]
My thanks to L. R. "Rosie" Brown for the request.