It's a pie melon, not a water melon. Once a fairly common weed on empty blocks, now you don't see them so much.
You can make good jam or relish out of them, and I've seen pioneer-type recipes for pie made of them (hence the name) but they're not really good eating.

Ouch ... the mother in law was looking forward to some sweet fruit .. LOL. Time to uproot these buggers and get something else in .. Thanks mate.

Usually with watermelons the seeds don't get dark until after they are full sized.

Pie melons aka paddy melons or pig melons are not terribly large, the biggest would be about the size of a large rockmelon/cantaloupe and on down to the size of a softball or so.
Melon, lemon and ginger jam is really quite nice as is melon relish but they're rather bland eating.