JBL's Charge 3 is a USB phone charger with a Bluetooth speaker built in. Or perhaps it's the other way round. Either way it comes with heavy-duty waterproofing, a whopper of a battery that'll let you siphon off juice if your phone dies, and you can daisy chain similarly branded speakers together to...
Sturdy build; Weighty sound; Decent waterproofing; Doubles as a portable charger
A little on the heavy side; Mid-range can be hit and miss