The Produce Blue Book asservates that watermelon isn't grown in the US from Dec-Mar. Why not?
This website says the same thing for the US from Dec-Mar.
Most melons (including watermelons) are warm weather crops. Watermelon needs consistent 80+ degrees F, 8+ hours of bright sunny days to get the energy to synthesize sugars for the fruit to be sweet. Most need 60 (small 5lb icebox varieties) to 90 days (10+ lbs) of full sun and heat to ripen. Some of the biggest varieties may need close to 4 months of summer weather.
Watermelons aren't grown because in the northern US, it freezes them to death--and in the southern US, it doesn't get enough heat and sunlight to grow well and be economical. It might be possible in Hawaii, or Guam or Puerto Rico, but their economies don't need it.