Watermelon help needed

So the four watermelons that have grown largest (from orange to cantaloupe size) have all developed black spots (and most split open at the black spot). Some grew on the ground, but the latest one was hanging out of a pot and not touching anything. Any ideas on what is happening and if it can be prevented?
I first wanted to blame the 100 degree weather a few weeks ago, but then the latest one seemed to have gone bad more recently - but I can't be sure. All input welcome!