must be able to be seen at depth
Ah! It's deep sea advertising


I have no specific knowledge but my 2p worth is:

Are you sure your mechanism would not run in a liquefied gas? If, as a result, you can reduce the required strength of the required pressure vessel, you might be able to include a larger power source (to keep the device running/ticking for the same period) for the same money/effort as a crystal sphere.

I think someone has already mentioned cast polymers; If you can live with a larger sphere and reduced clarity (not a problem if the output of your device is just a
semaphore) they would be worth looking into (no pun intended).