Combine simple syrup and watermelon in a blender; process until smooth. Push mixture through a fine mesh sieve; discarding solids.
Return watermelon liquid to blender and add lemonade and pineapple juice. Blend well and serve over crushed ice.
To make simple syrup: In a saucepan, combine water and sugar - cook over low heat until sugar dissolves. Bring to a boil; reduce heat and simmer one minute. Remove from heat and cool.