Watermelons and cucumbers should not be planted together since they belong to the same family and can cross pollinate which then leads to bitter tasting cucumbers and watermelon.
If the watermelons and cucumbers are planted close enough together then the vines can grow into each other and then cross pollinate with each other.
So I would advise planting the watermelons and cucumbers at a good enough distance away from each other to avoid them cross pollinating and causing bitter cucumbers.
I would plant the watermelons and cucumbers in different rows and around 10 feet apart away from each other.
You could plant the cucumbers and watermelons together but then you would not get very good cucumbers or watermelons because the taste will be different so plant the cucumbers and watermelons at least 10 feet away from each other to avoid cross pollination.