We currently have a blockrocker we bought on eBay. It's not waterproof but it rolls around with a handle and wheels built in like luggage. It is so loud and my husband also uses it as a portable amp when he wants to plug in his guitar. The battery literally lasts for days. We are switching over to play-fi though for whole house audio. It's like Sonos but cheaper and the different brands that offer play-fi all work together. They have stand alone speakers, sound bars and also receivers you plug into your current system. We are currently planning the outdoor areas as we finish up our AG pool build and yard makeover. We are goung to use the Phorus PR5 play-fi reciever plugged into a older Yamaha receiver A/B wired to either 8 outdoor marine speakers or 4 outdoor house speakers. A up by the house and B by the pool and future fire pit.
I assume you already know this but wiring multiple speakers to a single output (channel) is a tricky dance at best. You are playing of game of balancing impedance vs voltage, the result of which is not what most home A/V amplifiers or speakers are rated for. Review the attached discussion for some of the basics, but try to ignore the typical "thread static" emmiating between a few of the posters. The end result is usually a blown amp or fried speaker coils at worst and marginal sound output at best.
Why is connecting multiple speakers to one output bad? - Ars Technica OpenForum