I've only been making wine for a little over a year now, and like you, I was very eager to make watermelon wine as one of my first wines. I used about 15 lbs of my own muscadines and a couple of store bought melons to make a 3 gallon batch. I had no trouble during fermentation, but there was a pretty wicked smell coming from the wine as it aged. I scoured the internet for information about the odor and everything I found said it would go away with ageing. The odor is fading after about 6 months, and now I have a wine that tastes perfectly like watermelon rind. I have tried to mask the flavor with several different juices, and even went so far as to try and use the wine in a sangria. The rind flavor still comes through way too strong. I still drink a glass every now and then because I can't bring myself to pour it down the drain. Who knows, maybe in a few years...
I love watermelon. I mean, its one of my favorite foods. But i can honestly say I will never make watermelon wine again.
I am thinking about making a Welch's white grape juice wine and adding watermelon juice after fermentation though. That might get me the watermelon flavored wine I'm after.
Good luck, and enjoy!