DMW, I would like to join in with that.
I've been using the UE Boom and UE Megaboom for a few years and I'm actually quite happy with them.
Larger, powerful speakers are required especially in order to be able to play sound mock-ups of lower tones with sufficient volume (owls, rails, bitterns etc.).
And here comes a special problem with the UE Megaboom (and perhaps also with other speakers): this speaker does not deliver its full volume immediately, rather it only builds up after about a second.
When playing short, intense individual calls, this means that the UE Megaboom is in practice not louder than the smaller, lighter and cheaper UE Boom, because the full volume cannot be built up quickly enough.
Therefore I can recommend the smaller (and also waterproof) UE Boom, unfortunately not the Megaboom.
So if someone can recommend a large, waterproof and very loud loudspeaker for low tones that can immediately play at maximum volume, I am grateful for any advice.
Cheers, Stefan