That's not an excuse. Sailors can still fight, and all throughout our history, they were exptected to do both boarding parties and landing parties when Marines weren't around, which, considering how small the Corps was before WWII (back when it was actually a corps, and not a seaborne army), light infantry was a regular duty of American sailors.
I collect old Bluejackets' Manuals, and until fairly recently, light infantry training was a big part of the Navy's basic training. This is taken from my 1940 edition:
Just a few years prior, navy recruits also had to qualify with the Browning .30 cal machine guns as well. Also, keep in mind that support ships like the Lewis and Clark are no longer ships of the line. They're now US Naval Ships (USNS instead of USS), and are now largely staffed by merchant mariners on contract with the Navy. USNS ships can in reality hardly even be considered naval vessels the way we currently operate. They're just merchant ships with a coat of grey paint, for all practical purposes.