"Are you the lad for whom the pilot sent the boat?" inquired a man whom I afterwards found to be the second mate.
"Well, there he is abaft, in a P-jacket," said he, walking to the gangway, and directing the men to drop the boat astern.
I looked aft, and perceived my future master talking with the captain of the vessel. Philip Bramble was a spare man, about five feet seven inches high: he had on his head a low-crowned tarpaulin hat; a short P-jacket (so called from the abbreviation of pilot's jacket) reached down to just above his knees.
Frederick Marryat certainly uses an interesting word (and spelling) for what I'd call a peacoat. There's even an illustration of the garment in question, drawn by Marryat's friend Clarkson Stanfield for Poor Jack.
The P-jacket covers Bramble's thighs, as noted in the text, and how the buttons are arranged can't be seen. According to the wikipedia article for pea coats, a pea coat that extends to the thighs is actually a "bridge jacket" and is reserved exclusively for "officers and chief petty officers."
Philip Bramble is a maritime pilot, guiding large ships into harbors and up rivers, so it makes sense that he would have a pilot's jacket. Even his young apprentice Tom "Jack" Saunders owns a P-jacket.
I fixed my glass upon the vessel; and, after a time, not having forgotten the lessons so repeatedly given me by Spicer, I said, "She has no colours up; but she's an Embden vessel, by her build."
"Oh!" said he, "hand me the glass. The boy's right, — and a good glass, too. Come, I see you do know something; and good knowledge, too, for a pilot. It often saves us a deal of trouble when we know a vessel by her build; them foreigners sail too close to take pilots. Can you stand cold? Have you got a P-jacket?"
"Yes; father bought me one."
"Well, you'll want it this winter; for the wild geese tell us that it will be a sharp one. Steady, starboard!"
Some of the original watercolor illustrations for Poor Jack are in the National Maritime Museum's collection, and Marryat was known to be very fond of them. According to his daughter Florence, they decorated the room where he composed his last books. The accuracy of the nautical clothing can be assumed to be correct— Clarkson Stanfield was also a sailor, working on a merchant vessel before he was impressed into the Royal Navy during the Napoleonic wars.