You have two real options: Multi-zone or 70v.
"70v" uses a set of transformers on the output of the amp and the inputs to the speakers to match impedances. This allows you to connect numerous pairs of speakers to a single amplifier without impedance issues, and the higher voltages result in decreased power loss through the speaker wire. Furthermore, 70v systems can be equipped with localized volume controls cheaply and easily.
However, there is one downside to 70v, and it's a doozy: Sound quality. Unless you spend quite a lot of money on high-quality transformers, you'll end up with a sort of tinny sound. The best example of a 70v system is a supermarket muzak setup - reliable, cheap, reasonably loud, but not very good to listen to.
"Multi-zone" uses the opposite approach: Multiple small amplifiers. Using a separate amplifier for each pair of speakers means that you can have different audio source, volume level, and even equalization for each speaker. While sound quality is likely to be much better than a 70v setup, the cost will be much greater as well.
For the pool, I would strongly recommend using a standard pair of PA speakers (Yamaha, Madison, Mackie, Peavey, whatever) and a standard PA power amp (the Tapco J800 is powerful, cheap, reliable, and sounds good.) You'll save money on matching transformers, get better sound quality, and have the option of playing Mozart for the dentist's convention with Metallica at the pool party.
