Original poster here. I'm an oceanographer -- a physicist, not an ocean engineer, but I've talked with enough marine engineers to know about the issues. Designing instruments that operate unattended for long periods of time in the ocean without getting covered with barnacles, slime, worms, algae, and all manner of crap is one of the big unsolved problems in our field. Our best solution to the problem is to minimize the number of moving parts, and to keep most of the equipment well below the photic zone (the sunlit shallows where most of the life hangs out). Neither of these is possible for a hydrokinetic marine turbine.
(Random anecdote: another problem we have is that devices that carry electric current tend to get attacked by sharks, which have delicate electrosensory organs, so cables need special anti-shark armor.)
Another commenter mentioned corrosion: that's fairly easy to deal with using a sacrificial anode (http://en.wikipedia.org/wiki/Galvanic_anode). But biofouling is a lot harder to deal with. I'm willing to believe that the designers of this system have a solution, but only after they've successfully operated a turbine for several years without problems.
