Growing watermelons does not have to be reserved only for hot climates. There are varieties that can be grown in colder planting zones with a just a little help from a greenhouse.
If you are planting in a colder climate then it is better to buy ready started plants or start the seeds off indoors before you intend to transplant them outdoors. When the plants are in, use black plastic sheeting, that you can find at any garden center, around them like you would a mulch. The sheeting will help to warm the soil faster. You might also consider using a floating row cover just in case there is an unexpected late season frost. If you do use a floating row cover, make sure to take it off when any danger of frost is past.
Watermelons prefer a warm sandy soil. If you have a soil with a high clay content, use the black plastic sheeting to keep it warm. Watermelons need a lot room to grow. Their vines really spread out. Watermelons need to be planted in mounds, around 4 feet long, of soil laid out 6 feet apart in a row and each row must be from 7 to 10 feet apart.
If you are planting from seeds, put about 6 in each mound and plant them 1 inch deep. If you are using transplanted plants, put 2 to a mound and about 2 to 3 feet apart.
Watermelons do not need a lot of water, unless there is a long rainless spell. Their roots are deep enough to get the moisture they need from the sub soil. Just make sure you keep weeds away and watch out for the cucumber beetle who like watermelons as much as you do!