*Edit below due to retry*
I use strawberries instead of watermelons because I couldn't find any gelatin free watermelon jello. Other than the fruit and flavor everything else remain the same and the "pie" was very liquidy. Other than that it was still really good.
Edit: I re-tried it with the only changes (I think) being the brand of whipped topping (used cool-whip brand the first time and Walmart brand the second time) and I didn't put it on a pie crust. I don't think excluding the pie crust really did anything but I didn't want to use a pie crust this time just in case it didn't work. This time the filling did turn out to be a nice thick spongy texture.
