Leyland Cypress is an obvious choice but many gardeners get fed up with it becuase it can be weak and break easily. They also look pretty boring if you ask me. You could get enough small trees to surround your pool for a few hundred bucks, and within a few years, depending on the soil and sun, have a privacy hedge of 10-20 feet. I have 3 in my yard (sort of) that my neighbor planted 5 years ago that were 6 feet at the time and they are now easily 30+feet. This, despite the fact that they were planted in the middle of a massive patch of Japanese Honeysuckle. Then again the house where I grew up we planted some when I was 7 or 8 and they grew very very slow. I guess they do well given the conditions of my new neighborhood.