Combine watermelon and Simple Syrup in container of an electric blender; process until smooth. Push mixture through a fine-mesh strainer, discarding solids.
Return watermelon syrup to blender. Add lemonade concentrate and pineapple juice and blend well. Serve over crushed ice.
In a small saucepan, combine water and sugar; cook over low heat until sugar dissolves. Bring to a boil; reduce heat and simmer 1 minute. Remove from heat; cool.