In a small bowl, whisk together oil, apple cider vinegar, lime juice, lime zest, agave, salt and pepper.
In large bowl, combine pineapple, watermelon, cilantro and red onion. Toss with dressing and serve immediately.
Notes
Due to the porous nature of watermelon, this salad is best enjoyed immediately after assembly.
If you are pressed for time, you can cut the fruit and make the dressing ahead of time, keeping the watermelon, pineapple, onion/cilantro/jalapeño, and dressing in separate containers until ready to serve.
Any leftovers can be kept in an airtight container in the fridge for up to a day. Note that it will be very juicy if left for more than an hour.
Watermelon-Picking Savvy. Watermelons that are heavy for their size indicate that they will be super juicy. Look for dull skin and a creamy yellow patch where it rested on the ground. Shiny skin indicates that it didn't have enough time to ripen.
Pineapple-Picking Savvy. First, the pineapple should also be heavy for its size. Next, pick a frond from the center; it should release easily. Sniff the bottom of the pineapple—it should smell sweet. The exterior should be firm, but yield a little to pressure when you squeeze it.