In a pot, add water and cook the agar-agar and sugar till it boils. Set aside till it cools slightly.
Cut the watermelon in half. Use a spoon to hollow out the inside.
Cut half of the watermelon into cubes. Rest of the watermelon can be eaten.
Drain the juice from the watermelon shell.
Add your choice on fruits to the watermelon half.
Repeat the same with the other watermelon half, if using.
Pour the agar-agar mixture over the fruits till almost full.
Place the watermelon into a pan if necessary, and refrigerate for a few hours or till set and cold.
Remove from refrigerator and cut into slices and serve.