I bout this Rectangle Shade Sail for my patio. Actually though its for my plants. Its so hot he in Sunny California most of the time and it fries my plants. So I think this will work perfect. I had a smaller one on there but it was torn to shreds by the rain this year.
I'm think I ordered it a little to big though. So i had to cut it down to fit. But, all in all i think this will do nicely. 
It's lightweight and really light material so my Plants Hopefully wont get burned this summer.
If you are looking for one that is heavy duty. This ISNOT it! Like I said it's lightweight and will catch wind if its not hung up right. ALl in all though this is a pretty good tarp and I love the color, Matched my pool!