Another way to help solar heated swimming pools retain energy is by choosing a dark-colored bottom for the pool. As you've probably heard before (or noticed yourself), dark colors absorb heat. This is true not only for clothing but for pools. On the other hand, light-bottomed swimming pools reflect heat (if you live in a very hot climate, and you don't want your water to get too warm in the sun, then do the opposite--choose a light color for the bottom of your pool).
If you want a completely solar heated pool, you'll want to trim branches and shrubs to make sure as much direct sunlight as possible hits the pool throughout the day. However, you should leave some shady spots (perhaps on the northern side of the pool) that can provide a cool respite for poolside loungers who want a break from the heat.
Source: What Color Is Your Swimming Pool? A Homeowner's Guide to Troublefree Pool, Spa & HotTub Maintenance