Combine everything in a medium bowl. Let sit for at least 15 minutes before serving with chips.
It sounds weird... but it's not, I swear. Totally regular salsa here. If you're worried that the watermelon will be too sweet/fruity for salsa, you can use the parts closer to the rind to make it more tart... and eat the rest of the watermelon :)
Source of recipe: I wrote this recipe for the vegan chef battle =)