Add the watermelon, cucumber, red onion, jalapeño, lime zest, lime juice, and mint to a medium bowl. Gently toss until everything is evenly combined. (You can stop here and refrigerate for up to 3 hours if you're making it ahead.)
Right before serving, add the salt and gently toss again. Taste and add more salt if needed.
Serve immediately with tortilla chips.
Notes
Salt at the last minute. Salt draws the juice out of watermelon, so adding it early will leave a puddle in the bottom of the bowl. You can combine everything else a few hours ahead and just stir the salt in right before it goes on the table.
Taste it on a chip. Tortilla chips are salty, so salsa that tastes right off a spoon can taste over-salted on a chip. Season it the way you'll actually eat it.
If your watermelon isn't sweet, stir in a tablespoon of granulated sugar, honey, or agave.
Swaps. Chopped cilantro works instead of mint for a bolder, more herby flavor. Peeled and diced jicama works instead of cucumber if you want even more crunch.
How to store. Keep leftovers in an airtight container in the fridge for up to 3 days. It'll release liquid as it sits, so give it a good stir before serving.