Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

Cut watermelon into chunks, making about 2 cups. Puree until smooth.

In a large bowl, cream together softened butter and sugar until light and fluffy, about 2-3 minutes.

Add eggs one at a time, mixing well after each addition until fully incorporated.

Gradually add watermelon puree to the butter mixture, blending until fully combined.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gently mix the dry ingredients into the wet batter until no dry flour remains. Fold in raisins if desired.

Pour the batter into the prepared loaf pan, smoothing the surface.

Bake for approximately 1 hour or until golden brown on top, and a toothpick comes out clean.

Allow the loaf to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.