Ultimate Ears - Roll

Anything by Ultimate Ears. I have the Megaboom and a discontinued model called the Roll, both excellent. If size isn't a concern (if you won't be traveling with it much, etc) I strongly suggest getting one size bigger than you think you'll need. Trust me on this! It's always better to have more power than you need for audio. It's not just about volume, it's about the quality of sound at lower volume, too. Bigger is better. And they sell refurbished units on their website which is great, that's what I did. Same warranty and everything so why not?

The UE Roll was great

The best I’ve had is the Sony SRS-X5. But I should also admit I haven’t needed to shop for Bluetooth speakers since I’ve bought it 8+ years ago, so I have no idea really. I do have a UE Roll and a JBL Go and Go 2, but they aren’t comparable. They are in a different class altogether.

Ultimate Ears makes rugged speakers. I've had mine fall around a bit though never from a bike. I am not sure if any other company makes speakers that rugged. Tribit Stormbox Lava floats on water but it's huge for a bike. A UE Wonderboom or Wonderboom 2 might be better I think? Check them and the other speakers out. They're also mud resistant, have some shock resistance and float. The Wonderboom also has this lovely ring so you can hang it. Maybe they're just what you're looking for.