I've grown a varity called blacktail mountain that was developed in northern Idaho and is supposed to do well in cooler climates. Mine did fine in a not very cool summer and was earlier than the other water melon variety I had. I got them from Terratorial Seeds but other companies also have it.

Jonny's Selected Seeds from Maine might be a helpful resource. http://www.johnnyseeds.com/fruits/watermelons/ I haven't used them for years because the climate I'm in is so different but when I did I liked them a lot.