I am not sure where to post this question, I am guessing this could be the closest subject. Admin, please move as you feel fit.
I want to make an audible buzz from a submerged enclosure attached to a swimmer's chest. I am wondering if anyone can suggest a suitable device.
The only method I can currently think of is a voice coil activating a diaphram. Something like 'bone conduction' earphones, that I have used to listen to music underwater while swimming. The bone conduction earphones are in very close proximity to the swimmer's head.
Thanks 
What is "hearing" the buzz?
Try searching for "small underwater speaker". I found many different styles at a wide range of price.
Thanks Gilshultz
I did a Google search before posting here. Then I only found one item, which would be too big and too expensive at £74
.
I used the search phrase "small underwater speaker" and Identified a number of water proof speakers. Nothing that I think is suitable, the speakers are designed for coupling sound into air not water (very different acoustic impedance).
Steve
Hi Aarg,
The swimmer wearing the kit needs to hear the 'buzz'.
Steve
I would expect a piezo element to work underwater. You might have to coat it with wax or poly, something to waterproof it.
Thinking in a different direction:
A small pager buzzer motor. If it is mounted firmly in a waterproof case, the swimmer may be able to feel it.
Yes possibly This is intended for a training aid and I suspect there is a lot of 'flow noise', that would be greater than the buzzer motor.
I wonder if a helmholtz resonator will work, with a small water pump, i.e. a whistle. I found this online https://www.stormwhistles.com/. To whistle would require a small pump and I feel that is going down a different rabbit hole.
Oh BTW I thought the buzzer would be the easier part! 
Steve
Hmm, I wonder if a solenoid being operated would generate an audible 'click'?
Yeah, everything underwater is much harder. You have to think about odd conditions like water wicking in between multicore wire strands and causing hidden corrosion (long term), galvanic corrosion, and so on.
For solenoid clicks to be audible, I would imaging there would have to be a striking plate that would be free to resonate; possibly be felt by the swimmer's chest?
If the case is on the swimmer, vibrating the case in almost any way would work.
Perhaps a vibrating motor? Basically a small motor with an offset weight on the shaft.
Won't buzz but should be able to be felt.
Like in mobile 'phone handsets. I guess an experiment is needed at some future time
I was thinking more of an adult vibrating toy 
Should definitely get the swimmer's attention 
Remember the OP is placing it on the swimmer's chest.........
No, felt will get wet and deteriorate in time.
Punny, but easily misunderstood.
Just trying to keep the thread in the shallow end.
This will be used in a public pool with mixed genders and ages 
LOL