The real question is whether you want something that will flower, something evergreen, or something deciduous? Probably something evergreen because you don't want a whole bunch of leaves falling into the pool. When you say 16", do you mean that it can only grow up 16" wide or do you mean that the ground spacing is 16"? Can it grow out past 16" or does it need to stay enclosed in that space? If you can take a picture of the space you are talking about, it would be much easier for us to assess the landscape and which plant could be the most suitable for it. Also, what other plants are in the near area? Does it freeze in your area in the winter?