If they were seeds from a watermelon or cantaloupe that you bought at the market, the resulting plants are quite possibly some kind of hybrid, as you have know way of knowing what fertilized them and those sorts of plants are notorious for, well, let’s just say that if they are grown very near to a related plant they aren’t picky about what pollen fertilized the flower. First generation offspring are the normal, planted item but second generation (as in the seed from the first generation) will not breed true and can produce some interesting (and sometimes not very flavorful) results. Also they will not reliably produce the same plant in succeeding generations.