The navy has never had or still does not, have ranks " below decks"
It's important to realise, the Royal Navy is still one of the only armed forces that has this awful class system. Lower deckers were and still aren't allowed to fraternise with Officers. you were literally scum of the earth in those days.
Even today the officers still dress up for evening meal in the wardroom with servants (they are called stewards these days) and 3 or 4 course meals, silver service and brandy and cigars after dinner.
The Captain is NOT allowed into the wardroom, ONLY with permission of the 1st Lieutenant (pronounced LUFFtenant, most definitely not LOOtenant as that is the USA way of saying it). He must dine and take stock of his day, in his own cabin, which has all his own facilities. Bath, shower, day cabin, night cabin. He also has his own chef and own steward.
The RN still has these wacky traditions carrying on from many a year gone by.
Ratings are split into groups themselves such as junior (Ord,AB, Leading) and Junior Non Commissioned Officers JNCO(Petty, Chief and WO)
its an age old class system which probably will end up dying out when the military become more and more politically correct
This is just a little info for you
Below decks means literally, the lower rates.
Below officer RANK, everyone is a RATING
Those are the two descriptive ways.
Anyone from Junior right up to Warrant Officer or WO, are ratings
Junior
Ordinary
Able
Leading
Petty Officer
Chief Petty Officer
Warrant Officer
Above this the officers are Commissioned and they are ranked
In the 1700's they were different again these were a Status
They were
Servant
Seaman
Petty officer
Standing officer
Cockpit mate
Cockpit officer
Warrant officer
and above all those Commissioned officer
And in amongst all those were their position
https://en.wikipedia.org/wiki/Royal_Navy_ranks,_rates,_and_uniforms_of_the_18th_and_19th_centuries