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 next time, and plant in fields that didn’t have the disease the previous 3 years. The seed company should be told they are very probably selling poor seed. A fermentation process is known to reduce the seedborne bacteria to practically zero. The fruits that are not too far gone are still edible, although they may not be marketable.

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

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