The U.S. Navy has traditionally used marines for guarding sensitive locations. This includes nuclear weapons storage sites, and key bases. But over the years, the marines have been, increasingly, replaced with specially trained sailors. Since September 11, 2001, that process has accelerated. And in the process, the navy has built a large security force.
The navy has never had a security force ( Military Police) like the army and air force do, so the navy has taken its existing security specialist, Master at Arms, and expanded it from, a few thousand, to 10,000 personnel. These are now replacing marines in guarding key facilities. The marines will often still be there, but now the security detachments are typically be half marines, and half sailors (Masters at Arms). The marines are fine with this, given the manpower strains they are suffering because of the need to keep troops in Iraq and Afghanistan.