Green stew? Entry level deckhand @ 50

One option is the Royal Fleet Auxiliary, there are openings from apprenticeships to officers. Naval support vessels manned by civilians, it's an excellent way to build skills, experience & certs. You can join at any age.

https://www.royalnavy.mod.uk/organisation/rfa

I think your best bet would be the sail charter boats in the Caribbean, they are always looking for crew.
Would need to get some training first, such as Powerboat, STCW, etc. Have your partner take a cooking course or two.
I would imagine if you can get a license in short time (yachtmaster), the two of you could be running your own boat in a short amount of time.
No offense, but the two of you better be fit, personable and no baggage (kids, pets etc.) and ready to go at a moments notice.

Unfortunately the realization of the two of you getting a job on a megayacht is quite slim to be honest, so I suggest the above route.

Good luck

Ok, my opinion as I have been in the same boat a few times, as a Mate, and have also as a Captain on small to medium size boats, usually around Fort Lauderdale, Florida.

As a 50 year old, we tend to slow down and we don’t have the fire in the belly we had @ as a teen:
At 17 to 19 I was a deckhand, or rather an O.S. on commercial ships: Work hard, sometimes 17 hours non-stop with plenty of overtime on the books with 50% extra if night, holidays or whatnot.
Doing the same work @ age 5o is a no-go, too much manual labor, in the sun, or in the cold Artic, either way, too much work.
Same on yachts, always cleaning, scrubbing, polishing or just running around getting stuff and helping out: As a licensed Mate, or as a Bosun, you do less manual labor than a deck hand, but you work your way up in the meantime.
Fun when you are 21, less fun @ 31, 41 and 50.

I was also a fire fighter, in the Norwegian Air Force, but easy duty as there was no
Wars going on and very few crash landing, usually we sat at the fire station drinking coffee
and passing time until we went on leave, 1 week ON, 1 week OFF and I drove taxi cabs on my OFF weeks to make money for fun and to pay for civilian flight school in the US of A.
Been flying ever since, but again, I wouldn’t start as 51 year old deckhand..

Me too:
I was operating a 70’ twenty years ago, not charters, strictly private hence I had no stew and no mate: After every journey the Captain washed her and chamoi’d the gelcoat and SS railings, then cleaned the interior and inspected the engine room, etc.
Wasn’t too bad as we only ran when the owner was in town, in between I hired and fired numerous contractors as the boat was not commissioned right and there was always something wrong and it was my job to make it right.
Wish I had a young buck for a Mate, but the boss only approved a part time Mate for trips to the fuel dock, therefore we always washed and detailed the boat after going to the fuel dock.