From the Navy.mil page:
Navy and Marines. The Posse Comitatus Act proscribes use of the Army or the Air Force to execute the law. It says nothing about the Navy, the Marine Corps, the Coast Guard, or the National Guard. The courts have generally held that the Posse Comitatus Act by itself does not apply to the Navy or the Marine Corps. They maintain, however, that those forces are covered by similarly confining administrative and legislative supplements, which appear in the Department of Defense (DoD) Directive.
Thanks for that. I could not conceive of such a real loop hole