preheat oven to 400 degrees. add all marinade ingredients to a blender and pulse for 1 minute or so. if you don't have a blender, you can mix them in a bowl with a fork.
chop the watermelon into even cubes - keep in mind the cubes shrink, so don't make them too small.
separate the watermelon onto baking trays (make sure they’re not too crowded!) and lightly coat them with the marinade.
cook for 30 minutes then stir the cubes. add them in for another 25 minutes then check to make sure the texture is right. depending on the size of your cubes and your specific oven, this could take longer or shorter so continuously check up on them. it's better to overcook them than undercook them, so don't be nervous if they start charring - that's normal!
once they’re done, they should have a tuna-like consistency. silky, smooth, and buttery. add them into a plastic bag with the marinade, give it a good shake, and leave in the fridge for at LEAST 24 hours
after 24 hours, remove each poke one by one on a paper towel. use another paper towel to lightly tap some of the marinade off. you can serve as is, but I prefer the tuna with less of the marinade. serve in a bowl with all your favorite poke toppings!