Amends the Ports and Waterways Safety Act of 1972 to authorize the Secretary of the department in which the Coast Guard is operating to order any vessel to change heading and speed if the vessel does not comply with any regulation, law, or treaty or does not satisfy port entry conditions, or if the order is justified for safety.

Directs the Secretary to: (1) continue individual port and waterway safety assessments to determine the U.S. ports, waterways, and channels needing new or improved vessel traffic management risk mitigation measures, including vessel traffic service (VTS) systems; (2) identify infrastructure requirements to improve existing or support new VTSs; and (3) review and validate the recruiting, retention, training, and expansion of the VTS personnel workforce and require basic navigation training for VTS watchstander personnel.

Directs the Secretary to: (1) establish a medical review board to advise the Coast Guard on medical standards and guidelines for merchant mariner physical qualifications, medical examiner education, and medical research; and (2) establish and maintain a national registry of medical examiners qualified to perform examinations and issue medical certificates.

Oil Spill Prevention Act of 2008 - (Sec. 2) Requires all new U.S. vessels with a building contract date after enactment of this Act or delivered after August 1, 2010, and with an aggregate capacity of 600 cubic meters or more of oil used as fuel, to comply with a specified regulation requiring double hulls.

(Sec. 3) Amends the Ports and Waterways Safety Act of 1972 to authorize the Secretary of the department in which the Coast Guard is operating to order any vessel to change heading and speed if the vessel does not comply with any regulation, law, or treaty or does not satisfy port entry conditions, or if the order is justified for safety.

Directs the Secretary to continue individual port and waterway safety assessments to determine the U.S. ports, waterways, and channels needing new or improved vessel traffic management risk mitigation measures, including vessel traffic service (VTS) systems.

Directs the Secretary to identify infrastructure requirements to improve existing or support new VTSs. Requires a report to Congress containing recommendations for implementing the study results.

Directs the Secretary to review and validate the recruiting, retention, training, and expansion of the VTS personnel workforce and require basic navigation training for VTS watchstander personnel.

(Sec. 4) Establishes the Merchant Mariner Medical Advisory Committee and requires it to advise the Secretary regarding: (1) medical certification determinations for issuance of merchant mariner credentials; (2) medical standards and guidelines for the operators of commercial vessels; (3) medical examiner education; and (4) medical research.

Requires, in addition, that the Committee develop specific courses and materials for medical examiners listed in the national registry established under this Act.

Directs the Secretary, among other things, to: (1) establish, review, and revise medical standards for merchant mariners and requirements for periodic physical examinations of merchant mariners; (2) require each merchant mariner to have a current valid medical certificate; (3) conduct periodic reviews of a select number of medical examiners on the national registry; and (4) require each such medical examiner to, at a minimum, self-certify that he or she has completed specific training.

Directs the Secretary to: (1) establish and maintain a current national registry of medical examiners, requiring that the examiners be licensed physicians; (2) remove any examiner from the registry who fails to meet or maintain established qualifications; (3) accept as valid only medical certificates issued by persons on the national registry of medical examiners; and (4) include in the registry licensed physicians who are certified by the Secretary of Transportation to perform medical examinations of operators of commercial motor vehicles.

Authorizes the Secretary to make participation of medical examiners in the national registry voluntary if such a change will enhance the safety of merchant mariners holding U.S. Coast Guard issued credentials.

(Sec. 5) Directs the Secretary to conduct a comprehensive study and report to Congress on marine casualty causes and contributing factors. Requires the study to identify measures that will improve the ability of the department in which the Coast Guard is operating to evaluate future marine casualties, monitor trends to identify causes and contributing factors, and develop effective safety improvement policies.

(Sec. 6) Requires the Coast Guard's commandant to report to the Senate Committee on Commerce, Science, and Transportation on the need for tractor tug escorts to be used by vessels carrying petroleum products or with large supplies of fuel onboard in the five largest U.S. ports, by volume, where the use of such tugs by those vessels is not otherwise required by state law or Captain-of-the-Port order.

(Sec. 7) Amends provisions relating to the general authority of the National Transportation Safety Board to declare that specified provisions do not affect the authority of another department, agency, or instrumentality of the federal or of a state government (under current law, of the government) to investigate an accident under applicable law or to obtain information directly from the parties involved in, and witnesses to, the accident.

(Sec. 8) Requires the Commandant, to the extent practicable, to ensure that there is at least one trained and experienced pollution investigator on duty, or in an on-call status, at all times for each Coast Guard Sector Command.