You can buy loud Bluetooth Speakers
You can buy Bluetooth Speakers with an SD card
You can buy Waterporoof Bluetooth Speakers 

You can't buy one with all three features. The closest thing is Rockville RPB25 mine is over 2 years old and I own a Shaw 650, its not entirely happy with salt water, it gets in the buttons and they've been sticky for over a year. I struggle not so say "best possession ever" because is seems so frivolous, but its the best sub $100 possession ever there's a laptop, a Shaw, an RS600 a van an a landcruiser that were a lot more expensive.

You can take spotify and a phone and stuff 'em get a SD card put all the best tunes on in and get a Rockville RPB25, just press play. My new one is on order $70.25 AUD shipped and taxed 

And it always knows to play something deeply inappropriate when it passes a fleet of kids in minnows