use a hall effect device inside the pressure chamber which is sensitive enough
to detect a suitable magnet on a button. If the button/magnet assembly is
outside the pressure chamber then the activation force need not change between
surface and required depth.

Thinking about it while reviewing what I have written, it may also be possible
to do something similar optically - I do not know how thick the container needs
to be for 200 ft, but if it had suitable material then reflective opto sensors
where the led and optodiode are in a V looking for a reflective piece on the
underneath of the button may also work. The led could be pulsed on while sensing
the button. If more than one button is required this could also act as the
button select, with all the photo transistors wired in parallel onto a single
pin. :)

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