The day at sea was divided into watches, each watch being four hours long, apart from the two dog watches, which were only two hours each. (This allowed Stephen Maturin to joke that they were cur-tailed...) This gave seven watches, allowing the crew to rotate which hours they were on duty from day to day. The crew were divided in half (confusingly, into starboard and larboard watches). One day, the larboard watch would be on deck during the Middle Watch, the next night it would be the turn of the starboard watch.
Although all the various sentry posts had to be covered all the time, the Marines didn't stand watches; they could luxuriate in a treasured 'all-night in' when they weren't on duty overnight. Apart from the Marines, the only other people aboard who regularly had 'all-night in' were the idlers, such as the carpenter, sailmaker and cook, who worked during daylight hours, and the Captain, who was expected to be called at any time of the day or night if something was serious enough to demand his attention.
Until late in 1805, the ship's day ran from midday to midday, signalled by the middies and lieutenants taking the noon sight. (This explain's Jack's "Call noon, Mr Hollom!" in M&C; he was marking the start of a new day.)
The watches were:
Afternoon: 12:00 noon to 4:00pm
First Dog: 4:00pm to 6:00pm
Second Dog:6:00pm to 8:00pm
First: 8:00pm to 12:00 midnight
Middle: 12:00 midnight to 4:00am
Morning: 4:00am to 8:00am
Forenoon: 8:00am to 12:00 noon
Each half an hour, the glass was turned and the bell struck. There were eight bells to each watch, apart from the two dog-watches.
So, "Two bells of the forenoon watch" meant nine am.
The glass is not turned after seven bells of the morning watch (supposedly 11:30 in the morning) as noon will not fall at precisely the same time every day aboard ship, if she is sailing in any direction other than due north or due south. Instead, the order is 'make it noon' when the ship's officers and midshipmen are taking their noon sight, the bell is struck eight times and then the glass is turned. If the glass were turned at seven bells, there would still be sand left to run through when the sun is at its highest at local noon, if the ship were sailing west, or the sand would run completely out before local noon if the ship were sailing east.