Very informative Gav...thanks. The 50 m water depth really isn't a problem with current offshore platform technology. Over 15 years ago a simple, and relatively cheap, structure was designed to develop small Gulf of Mexico fields. One system was named Seahorse. A simple vertical steel column built in the shipyard and floated intact to the location. There it would be flooded until upright and driven into the seafloor. It was then anchored to the seafloor by a series of guide wires. The system was engineered to operate in water depths of at least 150 m. Just a guess but I suspect this system would be considerably cheaper then a floating spar. This system would seem to also eliminate the problems developed by a moving base. Perhaps even a more important factor is reducing installation costs. The equipment (cranes, support facilities) rented to install offshore structures is typically very expensive. These systems would probably have to be mobilized from a distant resource. The high mob and demob costs could be mitigated by installing numerous towers in a single phase. In addition to the Seahorse-like structures being installed in something akin to an assembly line, the turbines could also be mounted in a similar manner.

Such plans may already have been engineered, or are at least in the process. Earlier this year the state of Texas awarded a number of wind farm tracts of its southern shore line. I don't now the details but am fairly certain the water depths are at least as great as the one you mention for offshore Maine. I'll dig for some details on this project.