It was not good. It was pretty, but it was not good.
First, as I (theoretically) learned when making marmalade in January, when you're using citrus you need to soften it first. There was no softening step here.
Second, the recipe said to cut the lime in half, then slice it thinly. I did the best I could on the "thinly" part, but the end result was some big chunks of very hard lime. I love jam with a little texture--when I open a jar that has been reduced entirely to paste it always seems so joyless--but this was like chewing a cud.
Third, even though I reduced the limes in the recipe to 3, the ratio of pith to watermelon rind was off. I'm good with a bit of a bitter bite, but this stuff was so acrid that it kind of burned going down.
Fourth, the ratio of liquid to solids seemed to be off. I think of normal jam as fruit held in suspension by jelly. Here there was no jelly, just the fruit.
That all being said, the watermelon rind itself--i.e. the pieces of it that I picked out to taste--was actually very good. I'm considering trying again with some major modifications.
But I'm going to do a few batches of blueberry first. While it may not be as exotic, it turns out there's a reason I make it again year after year.