When in Bangkok and Samut Songkram, people are talking about floating market (or talat nam in Thai), they almost always mean Damneon Saduak. 25 km from Samut Songkram, this market has been recreated to show tourists a reality that is rare now in Thailand: the floating markets. Thais do not do their shopping here. Only tourists, Thai or not, go to Damnoen Saduak Floating Market and buy fruits, fried bananas and souvenirs. Do not eat breakfast before you come, you will have enough to eat there. Most of the trade is happening on the water but there are also some shops on the docks including restaurants. The market is held on a not very long canal but if you take a boat trip you can ask to go tothe other cannals. They are also interesting even if there are no vendors any more.
Damnoen Saduak has many critics about its lak of authenticity as it was recreated for tourists and its tourist overpopulation. Anyway it's still an interesting visit to do. Better if you mange to comme early (ideally around 8 am before the market is not very active) to avoïd jamming of tourist boats.