I saw a similar item for sale online and thought why not just print my own! Intended for scuba diving but is probably applicable elsewhere. Printed in bright orange PLA so if the worst should happen I might find it! Printed at 100% infill to ensure the chamber in the middle is airtight so if you do let go it should float to the surface. There's also a loop for a wrist strap (I stole one from my old Wii nunchuck). UPDATE: Turns out it wasn't airtight at all, down at about 7 metres or so for an hour on 2 dives it slowly began to fill up. I think the pressure differential caused water to be able to seep through tiny gaps between each layer (the water is still in there 2 weeks later!). YMMV with different printing technologies. SO! I designed a version with plenty of drainage holes, now available on the files page. Won't be as floaty but won't fill up :)