How will it cope with 5~10m high waves?
Curious fish? (like sharks, dolphins.. orca's?)
Also, it'd need some decent power to fight the currents.
But, to continue on the waves thing.. what about storms in general, and what about possible icing?
(planes do suffer from a lot of the same problems, but I reckon to a lesser extent.. the higher you go, the easier it is on the plane.. apart from the potential icing problem)
PS: both sound very interesting.. and very very challenging to get right 