I seem to recall (though no idea where, and can't look it up at the moment) that some "research vessels" in the deep sea were designed to "self propel" via a dive/rise method. Obviously, this removes any concern about capsizing if you know it will be diving periodically. But, does present other challenges.
This could be helpful for propulsion depending on what was required.
one called the wave glider is mentioned earlier in this thread. its tethered to its wings underneath. unless the tether is broken it is highly unlikley it will capsize.
i think the best solution to capsizing is to stay away from expected storms, bad times of year and sea monsters.
a forum for discussion relating to the FishPi is located at http://fishpi.org/forum/
at the moment its just me talking to myself, but hopefully things will get busier in the coming weeks & months.