He was actually a captain in Nelson's Navy, served under Lord Cochrane and saw a lot of action. His novels are the real thing, since he was actually there. He's a good writer if you are amenable to the style of the time -- but what I think is his very best book, Mr. Midshipman Easy, is written in a very casual, modern style and is also a sly indictment of socialism.
I never could stand the Hornblower books, the hero always struck me as a neurotic 20th-century man adrift in the early 19th century. Captain Marryat's characters are honest to goodness British sailors and true to their time and place. His portraits of some of the 'characters' before the mast are priceless.
All those good Irish soldiers and sailors demonstrated that they were willing and able to fight for England.