While the NTSB has yet to issue a probable cause determination, it doesn't seem too far fetched to suspect that the likely determination will be spatial disorientation, with a contributing factor being the conditions of flight (at night, over water). In fact, there are aspects of the crash that are reminiscent of JFK, Jr.'s accident.
The report indicates that the pilot-in-command held a commerical certificate. Although it is possible to hold a commercial certificate without an instrument rating, it's certainly the exception to the rule. So, for the sake of argument, let's assume that the PIC was instrument rated. And that makes the accident even more disturbing. One of the biggest criticisims of JFK, Jr.'s final flight was his decision to proceed at night, over water, without an instrument rating. This accident shows you that its possible for even an instrument rated pilot to lose control of the aircraft.
More information about the search for the wreckage here.