Dear all,
I am currently going through the process of applying to the RN and my recruiters are a Lieutenant (female) and a Petty Officer (male). As I am, of course, a civilian, do I address them as Ma’am and Sir, respectively? It feels too formal to address them this way, but also to informal to not.
Thank you in advance.
You certainly won’t go far wrong with formality and if they want you to address them otherwise they’ll say.
Though I would call the Petty Officer “PO” “PeeOh”.
It depends on the context, but for a start, never call a Petty Officer Sir!
You’re applying to join the military and dealing with them in their formal roles so using ranks is appropriate.
In writing or if you’re referring to them, use their ranks and names i.e. ’ could I speak to Lt Jones please?’. In a formal interview, an occasional ma’am or Petty Officer wouldn’t hurt and certainly at the end of the interview ‘it was a pleasure to meet you ma’am, Petty Officer’. But don’t over do it.
Given you’re presumably cadet given you are posting here, then it is absolutely appropriate to use Ma’am (or Sir), at least a few times, during an interview when speaking to a commissioned officers.
They will be aware of your involvement in cadets and it will demonstrate that you are aware of the rank structure whilst at the same time being courteous.
Agree with the above, but don’t overdo it (ie at the end of every sentence). What are you applying for? Best of luck.
Remember that Petty Officer is the equivalent of Sergeant in most other arms, but of course you already knew that 
NCO’s should be addressed as ‘mate’, and officers should be addressed as (firstly) ‘Tea, Two, Milky’ while pointing at them and clicking your fingers, and thereafter as ‘sport’.
HTH.
And drop in the occasional “Fella” with a four finger point and the occasional “cheers lofty”.