What: This group of smiling ladies worked as a side-party. Not 'party' as in drinks and dances, their job was to paint the side of navy ships visiting Hong Kong.
Here's a description of Mary Soo, well-known in this line of business:
The U.S. 7th Fleet, which plies its trade in the troubled waters of the Far East, has a girl friend named Mary Soo. Mary Soo, by tradition but not contract, has the garbage concession of all United States Navy ships entering Hong Kong, which out here is no small potatoes.
In exchange for the garbage, Mary Soo provides crews of Chinese women to paint the sides of Navy ships while they are in port. The Navy supplies the paint and Mary Soo does the work for free.
It's a happy arrangement for everybody. The Navy gets rid of its garbage, the American sailors who might have to spend their time chipping and painting the sides of the ships are free to go on liberty into Hong Kong, and Mary Soo, who has a market for refuse, is said to be the richest collector in the colony. 
Several of these small boats would row out to meet the navy's ships in harbour, and used as the platform where the ladies would paint from. You can see a bunch of bamboo poles tied up along the side of their boat. The ladies will tie rags to them, and use them as extended paint brushes.
When: Here's the back of the card:
Welcome to HMS Warrior. HK 1954.
Warrior was a British Aircraft Carrier at the time, and visited Hong Kong in 1954 on the way from Singapore back to the UK .
Although the photo was handed out as a welcome, I wonder if this photo was taken as the ship was about to leave. The ladies are dressed smartly, not in the paint-spattered clothes they wore to work. And one bamboo has been raised to hold a string of firecrackers, likely lit to wish farewell to the visiting ship:
Where: Victoria Harbour
Who: Here's the flag at the back of their boat, reversed to make it easier to read:
But that adds a bit of confusion to the story.
We've already seen that Mary Soo had the rights to work on all the American ships. But this is a British ship, and as far as I can tell a different lady claimed side-party rights to the British and Commonwealth ships visiting Hong Kong. Her name was Jenny, and she was known to so many British sailors over the years that her death was reported in the UK newspapers .
So if you know who "Mary Ah-Choy" was (or "Mary" and Ah-Choy" were), I'd love to hear from you.
- The Miami News - May 26, 1960
- HMS Warrior: Summary Of Service, 1946-1958
- Obituary for Jenny, The Daily Telegraph