Major marine casualty means a casualty involving a vessel, other than a public vessel, that results in: (1) The loss of six or more lives: (2) The loss of a mechanically propelled vessel of 100 or more gross tons; (3) Property damage initially estimated at $500,000 or more; or (4) Serious threat, as determined by the Commandant and concurred in by the Chairman, to life, property, or the environment by hazardous materials.