Like the others ..I just use fairy liquid (very occasionally) but always hose off in fresh water. Difficult small stains often get a wipe of petrol or white spirit ..as sometimes that works well especially if a tarry type stain. Its always well rinsed after. I have tried branded cleaners but found little advantage over my methods.
However I do recommend polishing the boat after cleaning it with a branded UV blocking product. Sun will eventually break down the PVC although it takes a good few years in the UK. At present I use poly marine inflatable boat finish. It also helps stop the red colour fading to pink if its left outside for long periods. Mine is permanently outside. Perhaps its not so important to use a UV block polish if you pack the boat away and only use it a few times a year though.
I scrapped my 16 year old seago and the PVC was still good after following my methods. My quicksilver is 13 years old and still looks great ..so I cant be doing them any harm ?
Photo of quicksilver taken last week..in terrible Scottish conditions