ABC’s submarine-themed drama may be close to scraping bottom. Fans of “Last Resort” — which features the crew of a ballistic-missile sub, nuclear warhead detonations, a fictional island in the Indian Ocean and a few treason charges for good measure — got good news last week when the network ordered two more scripts in addition to the series’ planned 13-episode run. But an extended script order isn’t the same as a renewal for a second season — it doesn’t even guarantee those extra shows will be produced. And it doesn’t address the show’s ratings slide: Thursday’s show scored a 1.3…