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.
The letter is clear. He needed the sailors to get the provisions to shore. If all he needed were the Powhatan's guns, he would've simply said he needed the Powhatan, not "300 sailors". What's the use of sending the Powhatan if you're not going to send sailors to man the guns? Your theory is not logical.
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.
What's so complex about it? We wanted to send provisions to shore, Lincoln knew that as soon as he tried the Confederates would attack so he sent them to Florida instead. Better to do it that way than to lose both.