I've got a batch of watermelon in secondary right now as well. This is my first batch of wine (didn't realize it was such a difficult one. Watermelon was just cheap! Lol) I sulfited and then used Red Star champagne yeast which probably wasn't the best for preserving flavor and color, but shot through fermentation quickly at 65 degrees, so I made it through primary without spoilage.
Acid content has been the most difficult variable for me. I didn't have a pH meter when I started, so I used a TA titration kit to get acid in the ball park. I had to add what I thought was an awful lot of acid blend (don't have my notebook handy) to get in an acceptable range. I ordered a pH meter and was prepared to have to readjust it back up after primary since I figured I over did it. When I checked pH while racking about a month after primary, it was 4.0.... This kind of blew me away. I corrected it back to 3.5 with tartaric to a taste just a bit more tart than I want in a final product. The pH change made an amazing difference in taste. It's still not fantastic as is, but in a few months and with a bit of back sweetening, I'm hoping for something descent for a first attempt.
I used super kleer to clear and it looks brilliant now. There isn't really any pink color to it, but I think I have my yeast to thank for that. Next time I'll use a different one or add watermelon to secondary. Next week, I'll add sorbate, backsweeten, and prep for bottling.. I'll have a glass or two then and update.