Have you seen the link on Facebook about how to choose a juicy watermelon? I wish I could go back and find it to link to the proper source, but to go back and find ANYTHING on Facebook is a nightmare. Anyway, when picking a watermelon, look for the ones with a yellow spot like this:
I would avoid those watermelons, thinking they are under or overripe, but now I know better. The yellow spot means that the watermelon was left on the vine longer, long enough to obtain that spot, and the longer on the vine, the more ripe (and juicy) the fruit. Also, if the bottom of the watermelon looks slightly sticky (couldn't find a photo of one that does), that is desirable because it is because of the sugar in the juice.
So I happened to have a watermelon with a yellow spot when I found this information and, sure enough, it was really juicy and delicious. Three cheers for the ugly yellow spot!