To become a Pilot/Observer aircrew in the Royal Marines, you need to pass Flying Aptitude Tests (FATs), Pass the Admiralty Interview Board for officer, much the same as Pilot/WSO in the RAF must pass FATs & Officer Selection.
Have a look at
Royal Marines Officer Rates of Pay, scroll down then look at flying pay on the right. As a RM Officer as a Captain, you're on £34k anyway, as Pilot/Obs you get paid just under (UKP) 5k a year Flying Pay, which increases incrementally at around (UKP) 1k per year (ish) to a maximum of (UKP) 15k in addition to your own wage.
There are some Other Rank RM Aircrew, who also get Flying Pay, but are no longer recruited as NCO pilots, just aircrew ("Dopes on a Rope" malarkey). There are a
few (ie: VERY FEW) RM rotary wing pilots with 847 Naval Air Squadron flying as pilots & navigators on Lynx helo's. There are
some Royal Marine fixed wing (fast jet) which number less than the fingers on Anne Boleyn's starboard mitten.
If you want to be a fast jet WSO(RN Observer), the RAF have more vacancies- not that it's any easier. If you want to be a Helicopter pilot, the RN have more vacancies. Becoming a RM Pilot or Observer is probably one of the most difficult trades in which to specialise in numerical terms. If you haven't already, you need to talk to your local AFCO before you consider RM aircrew- it is most certainly not easy.