Cut the watermelon into 2" squares, discarding rind and any large seeds.
Toast the marshmallows either on skewers over a grill or on a parchment-lined baking sheet under the broiler, about 1 1/2 minutes or until golden-brown. Place them on top of the watermelon squares and smoosh them flat with a utensil.
Combine the carob and coconut oil in a small bowl, mixing well. Add more carob powder or a drizzle of honey to taste.
Drizzle the "chocolate" sauce over the marshmallows, then serve.