In a small pot, combine water and sugar and bring to a boil. Boil until sugar is dissolved, then set aside to cool.
In a food processor, puree raspberries and watermelon. Add lemon juice and cooled water.
Add cool whip and puree until combined.
Place in a freezer safe container in the freezer. Freeze until desired consistency.
Notes
*Optional: Stir every 1-3 hours to prevent it from getting too icy. I froze mine for 7 hours, stirring about 3 times, and then placed back into the food processor to get a creamier texture. You can skip these steps, but they help achieve a creamy consistency.