Yeah, from that Jack Keller page it seems like the biggest challenge is the quickness with which watermelon juice spoils. Sourcing good fruit it probably an issue to, but it seems like you'll be getting good product down your way so that's less of an issue.
To get something cider-ish I would follow his recipe, omitting most of the table sugar. Take a hydro reading and if it has an OG somewhere in the vicinity of 1.050 I'd go ahead without any sugar. If it's lower, add some until you get there. I wouldn't mess around with the acid blend until you bottle or omit it entirely
I'd go with one of the ale yeasts recommended for cider making on these threads. I like Nottingham.
It will ferment out dry so consult one of the many threads on here that detail how to get the finished product that you want (dry-flat, dry-sparkling, sweet-flat, sweet-sparkling). Those instructions will be the same for any fruit you use.
JS