Like the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, the Montara Commission of Inquiry, the Norwegian government, and leading safety and regulatory authorities around the world, the IADC recognizes the risks associated with complex, multi-agency regulatory regimes. A single authority should be responsible and accountable for safety and pollution prevention at offshore facilities, and should draw on the expertise of other agencies and organizations as necessary to achieve performance objectives.
The safety and environmental risks associated with fragmented or compartmentalized regulation include gaps, overlap, confusion, inconsistencies, and conflicting standards. Industry and governmental personnel spend too much time coordinating with multiple parties and not enough time managing safety and environmental risks.
Link to IADC comments. Key quotes:
IADC continues to be concerned by seemingly duplicative regulatory requirements imposed by the Coast Guard and BOEMRE, particularly where the agencies appear to have divergent views regarding the placement of regulatory responsibility.
One cannot holistically address safety when faced with the unyielding and overlapping demands of multiple narrowly-focused regulatory agencies.
Bud's Offshore Energy (BOE) 
As alternative, and one that doesn’t require (possibly contentious) legislation, is for the agencies to work togehter an leverage their existing authorities.
There are other agencies, besides the Coast Guard, having regulatory authorities affecting health, safety and the environment on the OCS: EPA, FAA, CDC, FCC, NOAA, DOL, etc. Which of these would BOE exclude from the functions of the super-agency?
Cooperation among agencies and leveraging of resources has always been US offshore policy (at least ostensibly) and has worked, particularly at the district and regional level. However, agencies have different agendas and priorities, and effective coordination has not been sustainable. Some have proposed some type of OCS coordination authority, but this could just add another layer of government and complexity.
As I see it, the single offshore regulatory agency would only address safety and pollution prevention. Environmental science, endangered species, and related responsibilities including the EPA discharge programs would remain separate. Including these functions would be too complex and divisive, and it’s probably best that environmental programs be independent from the operational agency.
The regulatory program of BOEMRE, the offshore facilities responsibilities of the Coast Guard, and the PL safety duties of PHMSA are the only functions that would be merged. This would cover almost all of the OCS safety and pollution prevention functions.