I agree with russ.
Pressure can easily be converted into useful energy. This has not been a problem for some time now.
To make it work, however, one must remember that the source of a pressure MUST propagate from a higher to a lower pressure environment in order to extract energy.(Think of hydroelectric dams, for instance)
In an underwater colony, one could easily designed a portion of the shell to perform a "controlled collapse" from water pressure, and thereby convert to useful energy. However, it is short-lived, and to return the system to a further potential state would require as much energy as was extracted. So, it would be a "one-shot" affair.