It's a century later but the Battle of Diamond Rock has a great story that may be of interest.

The British had set up Diamond Rock as a stone frigate, and used it to harrass french shipping entering Martiniques main port. The French decided to take action against HMS Diamond Rock, so went to set up a battery with which to attack.

However, a group of local slaves sailed out to the rock to warn the British. One requested amnesty under the British crown, so Hood offered him a deal - Lead a party to the location where the engineer was staying, and he could enter the Navy as a free man.

Once they had captured the only French engineer in the area, the french had to turn to the far more costly effort of blockading the island and assualting it.

What was expected of commoners who wanted to join the Navy? What was the process of joining?

Being in the wrong place at the wrong time; for both of them.

No, literally. Gangpressing, the forceful recruitment of people into a navy, was quite common in the British navy during wartime (it happened in most European navies of the time, apart from the Dutch Navy). If you were between 18 and 55, and had some seafaring relevant abilities (ie; if you were a merchant sailor); then you could be forced to join the navy against your will. You didn't want this to happen, because the pay was crap. Volunteering was better, because you'd get a bounty upon joining, but it still wasn't an awesome job to have; the desertion rates between volunteers and those pressed into service was practically the same.

As for officers; the upper ranks were pretty much exclusive to the gentry.