Amphawa is immensely popular among Thai visitors from Bangkok, so expect all the hotels to be sold out in weekends. If you really want a room during this time, it is absolutely necessary to book at least a week in advance. If you haven't done this and still want to stay over, you might want to try one of the so-called "homestays" in and around the floating market. Or you can visit as a day trip from Bangkok, as most people do.
There are decent hotels in Samut Songkhram City, some 6km distant. See Samut Songkhram listing for details. A blue songteow does the trip on a regular basis.