Chop about 2 cups of seedless watermelon into pieces, then blend until smooth to create a puree.

In a bowl, combine 2 cups all-purpose flour, 1 cup sugar, and 2 teaspoons baking powder. Whisk until well blended.

Cream together ½ cup softened butter and 2 large eggs until light and fluffy, then mix in the watermelon puree.

Gradually add the flour mixture to the wet ingredients, alternating with the watermelon puree. Mix gently until just combined.

Fold in ½ cup of raisins or chocolate chips if desired.

Preheat the oven to 350°F (175°C), grease the loaf pan, and pour the batter into it. Bake for 45 to 55 minutes.

Let the bread cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.