I came here to get an answer to the same question. The concern is not necessarily for seed saving, but for a quality fruit. When watermelon and cukes cross-polinate, you get cukes with tought rinds and watermelon that tastes rather like cukes. I don't know why, they are different species of the same curcurbita relatives, but I read and heard it from enough unrealted sources to be concerned.
I started a garden with help from a neighbor, and planted cucumbers. Now he wants to add watermelon in a separate raised bed. I need to know how far I have to locate the melon bed from the one with the cukes, so I don't get unappetizing produce.
If it was only prettiness, I would not care! Some of the best tasting stuff I ever got from a garden was nowhere near pretty enough to sell; fine, 'cuz I want to eat, not sell!
Somonee suggested it is less distance than bees, but distance does affect bees. Anyway, would it help if I had lots of other kinds of plants for the bees? I've got arugula, allyssum, beans, carrots, an eggplant, kohlrabi, lettuce, marigolds,okra, peas, a pepper, a sunflower, tomatoes, yarrow, and at least a dozen different herbs. There are to be some kind of trailing flowers in planters atop the corner posts, too. Are there any specific plants that would minimize cross-polination?
For the newbie who is confused already, another new concept; companion planting. Some plants make other plants happier, like marigolds do for tomatoes and for roses. Others just fit together well in the same space, like "the three sisters," corn, beans and pumpkins, as "Native Americans" were doing before whites set foot in the New World. It is a lot to wrap your head around, but if you make it work for you, it makes the garden produce better, which is easier in the long run.
