So you admit that they exist. Good.
Marines exist, there's no doubt. They do not, and have never, performed the task which you describe. Had he desired Marines, he probably would have used the word Marines. You are fundamentally confused about something; the letter is talking about sailors who would man shipboard guns - crew serviced weapons, not shoulder-fired. These are the men who would be needed to shoot their way past the shore batteries to get to Sumter.
Had they done it, odds are the public would not have stomped thier feet and screamed, "It's ours, ours ours ours, and we want it back." The events surrounding Sumter appear very carefully crafted. It darned near takes a program to keep track of who's shafting who.