One of the more authentic floating markets is in Amphawa, on the Mae Klong River in Samut Songkhram Province, Thailand. I understand it is only held at a certain time of the month, in line with the lunar calendar, probably due to the tides on the ocean. I don't really know more than that, okay?
What I do know for sure is when I visited Amphawa in 2002, there was regular everyday trading going on from boats. Merchants were using boats to cart produce around. One guy was selling ice cream from his boat.
And I ate noodles from a noodle lady in a small boat. She cooked with propane but paddled her boat around, searching for hungry people and selling noodles. I don't remember how much the noodles cost, but they were cheap.