] I was surprised - were the naval punishments of JA's time really that severe? Apart from death or being "placed in irons", if a sailor misbehaved, depending of course on what he did, what other punishments could he expect?

Mutiny or striking a superior officer could indeed get you hung. The ordinary naval punishment for sailors -- and it was a routine occurrence -- was flogging with the cat o'nine tails, sometimes into the hundreds of lashes. Offenders were ceremonially flogged in public, before the assembled crew, at the command "Hands to witness punishment." The offender was trussed up to a hatch grating and flogged by one of the bosun's mates. Midshpmen (i.e. teenaged officer candidates) were bent over the breech of a cannon and caned on the buttocks; this was called "kissing the gunner's daughter."

] What sort of rank is a post captain, what does he do and why is it named as such?

A "Post Captain" is the equivalent of the modern rank of Captain RN or USN. He's the commanding officer of a "post ship," which was a ship big enough to have a "sailing master" as well as a commanding officer. This would be a frigate or larger vessel. The commanding officer of a sloop of war or smaller, which didn't have a master, would have the rank of Master and Commander, which is the modern Commander RN or USN.

A Post Captain was the commanding officer of a major warship, with almost absolute power over his crew. Once "made post," he would move upwards by seniority and, if he lived long enough, automatically become an admiral. There were more captains and admirals than there were commands for them, so the Admiralty got to pick and choose who went to sea and who stayed on half pay, based on a combination of talent and patronage.

The "sailing master" goes back to the days before a professional navy, when civilian merchant ships would be converted into warships during wartime. The crew had two components -- the merchant sailors, under the command of the master, and the fighting men, under the command of a captain. The master was a professional sailor, the captain, who had overall command, was a gentleman. In the English service, the two sides began to amalgamate in Elizabeathan times -- Drake said that "the gentleman must haul and draw with the mariner, and the mariner must stand and fight with the gentleman." By the 18th century, gentleman officers were trained seamen, and aliterate seaman might "come in throught the hawsehole," being promoted to master's mate and eventually passing the lieutenant's exam and obtaining a commission.

The sailing master was a vestigal remnant of the old division between seamen and gentlemen. He, the purser, the bosun, the carpenter, the gunner, the sailmaker and the cooper were "standing officers," appointed to the ship by the Navy Board. These were all technical specialists -- the master was the navigation expert. The commissioned officers were appointed by the Admiralty.