I live in Barnes and used to live directly on the river.
I have used 999 and asked for the coastguard twice and they just put you through so it does work.
You then tell them there is a problem and can they launch the Chiswick lifeboat which in my case they did in a few minutes ( I could see Chiswick pier out of the window!)
Of course in the middle of all this my wife is shouting across the water that help is on its way and at the same time reporting that only one chap can now be seen - no pressure then as I hung up from the first 999!