I don't think it is a watermelon. I see many that look like this on the market today. The base color is green with a top coat of pink. The pink is usually on all raised areas of flesh. It is a no name chalice but a large colony is amazing with the random look of the raised pink markings.
what did you buy it named as? thats the only way to find out if it is named without linage it is just another chalice....
but does look like a lagoon chalice...