Not trying to dismiss your advise but it seems like the jelly fish are in salt water when you get stung. If you are immersed in salt water, how does "rinsing" with salt water make any sense.
If you are at a beach in a well populated area where medical attention was close it might make sense. But on a boat in the Caribbean, going to Peebles is the last resort for me.
I have been stung and vinegar worked fine.