Not sure what your climate is like there at Prince Edward Island, but I'm guessing that part of your problem is that your growing season is a little short for warm weather, slow maturing things like pumpkin and watermelon. I looked it up ( https://www.gov.pe.ca/weather/annual.php3 ) and it looks like you probably have frosts in to May and starting again in Oct.
"Pumpkins are a warm-season annual that require from 90 to 120 frost-free days to reach harvest. Sow pumpkins in the garden in spring when all danger of frost has passed and the soil temperature has reached 65Ã‚Â°F and night air temperatures are above 55Ã‚Â°F. In cool-summer regions grow smaller varieties." https://www.harvestwizard.com/2009/03/how_to_grow_pumpkin.html
So you can't plant it out until the soil has warmed up and the night time temps are in the 50's and after that it needs at least 90 frost free days maybe a lot more, depending on variety etc. And watermelons are similar in being warm season plants that can't be put out til the soil is well warmed up.
You can help them along by starting indoors, to give them a head start.
You could also think about in early spring putting some clear plastic over the bed you want to plant them in, help warm the soil up earlier.
One of our regular contributors did a really nice piece on companion planting, here:
I don't think it talks about watermelon, but it does mention pumpkins in the bottom section.
Companion plants are plants that benefit each other. One could be a trap crop that draws pests away from the other, or could be insect repellant, or gives off nutrients that the other wants, etc.
You don't have a great climate for pumpkins and watermelon, but you do have a great climate for all the cool weather crops - broccoli, cauliflower, spinach, chard, carrots, onions, lettuce, etc... You could work real hard to get a couple watermelons or you could concentrate on things that will like your climate! (Just a thought)
PS the pumpkins and watermelons are heavy feeders that need lots of soil fertility and LOTS of water.