In a medium salad bowl whisk together the Tamari sauce with the lemon juice, sesame oil, garlic, ginger, scallions, chili flakes and sesame seeds. Add the watermelon and sea vegetables and mix to coat well. Cover with a lid and refrigerate for 3 hours.
After 3 hours have passed heat up a large skillet over medium low flame. Use a slotted spoon and transfer the watermelon from the marinade into the skillet. Saute until most moisture has evaporated and the watermelon cubes have softened, about 10 minutes. Transfer it back into the poke marinade, cover with a tight lid and refrigerate overnight or at least for another 2-3 hours until completely chilled.
Serve the poke sprinkled with the fresh cilantro, chives and extra sesame seeds on top.
Watermelon is one of the healthiest foods on the Planet, it has a high water and lycopene content plus it is loaded with Vitamin A and carotenoids.