Are you trying to screen sunlight over the water itself, or over an adjacent deck area? While some trees are known to drop a lot of trash, all trees drop some trash some time. So placing one where foliage grows over water is asking for it. When I think of 'cleaner' trees, Japanese Maple comes to mind, but it's not exactly known for fast growing shade. It makes nice shade, but it takes a while. Though it's not an answer to the question you asked, shade from a manufactured source (shade sail, etc.) would probably be a better solution for shade near or over a pool. And it could happen much more quickly.

Good ideas. I would like natural shade, but it's going to take awhile to grow a tree to the proper height. I love the look of Japanese Maples, so I might try that, and just keep it at a safe distance from the pool to minimize trash.