Watermelon is a different species to other melons and has seeds in the flesh, rather than separate to it. Modern varieties have fewer seeds. Buying a whole watermelon can be a bit of a gamble. Tap it around the sides and it should sound hollow and the tapping a little muffled. The best idea is to get it cut open for you so you can assess the colour and texture. Redder watermelons tend to be sweeter than paler ones. The texture should be firm, not like soggy cottonwool. Once cut, watermelon will seep juices, so store it in a container in the fridge.
All melons are made up of about 95% water, making them a refreshing low-cal snack. They are rich in potassium and have good amounts of Vitamin C, calcium and phosphorus. Watermelon contains vitamins A and C, and some calcium, plus significant amounts of phosphorous.
There are plenty of watermelon recipes on the Shared Kitchen site.