From the photos of the fruit, it looks like fruit blotch, caused by the bacterium Acidovorax avenae subsp. citrulli. This is a seedborne disease, and the pathogen survives well in the soil and is capable of attacking a number of cucurbits. There is little to do except switch to another source of seeds, and plant in fields that did not have the disease in the previous 3 years. The seed company should be told they are probably selling poor seed. A fermentation process is known to elimminate the seedborne bacterium. It may still be possible to eat the flesh of the fruit, although the fruit are unlikely to be marketable.

The bacterium used to be called Pseudomonas pseudoalcaligenes. There has been extensive work on the disease in the USA in recent years. Key workers for an Internet search are Schaad N for seed detection methods; and Walcott R and Gitaitis R for epidemiology/control.

A Google search will give many leads. Seed infections are a main source of disease and spread internationally.