The first time that you venture out east in Harvest Moon: One World and step into the desert beyond Halo Halo, you’ll succumb to the heat and faint.
You’ll then awaken in the Animal Shop of Pastilla, needing to help Jamil acquire three Watermelons to appease Saeed’s demands.
So, here’s how you can go about getting the three Watermelons that Jamil, as well as where you can find Watermelon Seeds for future farming.
Where to find Watermelon Seeds in Harvest Moon: One World

Watermelon Seeds are fairly easy to find when you know where to look, with them spawning throughout each day in the same location.
To reach the Watermelon Seeds location shown above, go from the General Store in Halo Halo, on the beach, follow the path leading west and take your first left.
You’ll run between two Coconut trees and find as many as three Harvest Wisps down the route. One of the Wisps in the opening that overlooks the lake will give you Watermelon Seeds.
So, you just need to return to the Watermelon Seeds location on a few occasions and then grow the Watermelons on your farm.
Tips for growing Watermelons in Harvest Moon: One World

Watermelon Seeds aren’t too difficult to grow, and as long as you’ve got what you need to avoid fainting in the desert, you can even plant them in their preferred arid region.

Once you plant the seeds, be sure to water them each day and put down some fertiliser to protect them from extreme weather conditions. Four days after planting the Watermelon Seeds, you’ll be able to harvest a Watermelon.
On the list of the best Seeds in Harvest Moon: One World, Watermelons rank quite low as they take four days to produce one fruit that’s only worth 100G, so completing Jamil’s first quest doesn’t hit you financially.
With three Watermelons in your Bag, you can return to Pastilla, meet Jamil in the Animal Shop, and hand them your newly-grown fruits.

While the Watermelon has little monetary value after you’ve completed this quest for Jamil, it can mutate into a Cannonball or Mellow Yellow by planting it on the Halo Halo beach (Mellow Yellow) or Calisson plains (Cannonball) during the spring.
So, even though Watermelons are primarily sought to complete Jamil’s first quest, as the fruit can mutate, it’s worth collecting more than three Watermelon Seeds in Harvest Moon: One World.