Watermelon dehydrates into a taffy like candy. Key to the process is making sure to flip it before it sticks to the racks too badly. You’ll want to use a seedless watermelon because it slices into firmer pieces and you don’t have to deal with any/many seeds.
Cut watermelon (flesh-only) into strips about 1/4″ thick by ~3″ wide and no more than 6″ long. As the watermelon dehydrates, it gets nearly paper thin and hard to handle in larger pieces.
Drying time at ~160F was around 14-16 hours, longer of course for lower temps. A half of a large (~14″ oval) seedless watermelon dehydrated down to easily fit in a sandwich-sized plastic bag, so shrinkage is extreme. See picture below as example of typical dried piece size and quantity for half a watermelon. Fruit basket in background contain tangerines for scale.
Once the watermelon is dried, place it in a resealable plastic bag immediately. If you leave it out it will absorb moisture and rehydrate into a thin slimey gooey mess. You may want to consider lightly coating pieces in powdered sugar or cornstarch to keep them dry and from sticking to each other.
Cleaning tip – If watermelon does stick to your racks simply leave them out a day and they will be much easier to clean. You can also use this trick if you don’t flip it in time as it will be a lot easier to peel it from racks, then continue dehydrating.