I hope it works out for you! In my research, I have found that boiling the puree or using the rind gives it a beet like flavor and possibly a vinegar like flavor. I guess it is all up to the beer gods at this point!
For 1 gallon, I would do 1/4 of a watermelon. For 3 gallons, I would do at least half a watermelon. DO NOT boil the watermelon or use the rind at all!!
Best of luck!
I did a lot of research about watermelon recipes before settling on this kit from Adventures in Homebrewing (AIH) (http://www.homebrewing.org/AIH-Fruit-Wheat-Recipe-Kit_p_541.html). I do want to say that I am not affiliated with AIH in anyway, but do like their kits and almost exclusively.
I...