Another idea - put an accordian shaped balloon (shaped like the boot on a
shock absorber) in the top of a (vertical) cylindrical tank to displace the
water.  Pump your air from the storage tank (compressed) back and forth to
the balloon in the ballast tube (expanded).  A string from the end of the
balloon and some pulleys then drives a rotary encoder (needs a spring loaded
take up reel at the other end).  [Could be mounted inside the balloon if
water is an electrical problem].

A completely closed (air) system should be more reliable than one where your
compressed air gets to contact the water.  Or, for more realism, use a
largish cylinder of compressed air (enough for several re-surfacings) and
just dump the air from the ballast balloon out some one-way valves when
diving - should look spectacular as the sub drops below the surface with
jets of air/water shooting out of the dive valves.

Bye.

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