As far as Bluetooth speakers go, the UE ROLL 2 is probably one of the best, at least if you're aiming at under $100. However, now you can score one for $68 at Amazon and Wal-Mart, provided you're OK with grey or blue.
The UE ROLL 2 is waterproof, with IPX7 certification - which means it can be immersed in 1m water for up to 30 minutes - and can go for 9 hours on a charge. The speaker is in a disc shape - think of a throwing discus and you're on the right track - with buttons up top for changing the volume and a bungee cord stretching round the back, with power and Bluetooth powering buttons too, as well as a MicroUSB port and headphone jack with a flap for waterproof-ness.
Since this thing is waterproof, it's ideal for a pool party, or a day with friends at the lake or beach. On Amazon, the grey - or 'Volcano' - edition is available for $68; the other colors are still the standard price of $95.99 (or thereabouts). At Wal-Mart, there are exactly seven blue - 'Atmosphere' - models for $68.99, with grey at $67.95. So, if you're particularly keen on having a blue Bluetooth speaker, you'd better be quick.