WOW-SOUND 17-9000 Bluetooth Floating Waterproof Speaker, $147.45 at Amazon (originally $208.77 — you save 29%)
This is the be-all and end-all of outdoors speakers within most reasonable budgets. It floats, it fits into a cup holder (which means, like, in your cooler seat, in your console, in your pool toys, or pretty much anything with a cupholder). Plus, the 360 degrees of LED lighting comes in handy as one might imagine, and the company claims that it'll hold a charge for over 50 hours of playtime. Oh, and it sounds pretty darn good with a four-inch passive bass speaker and omnidirectional Hi-Fi sound. You get the idea. It's worth it.