One of the easiest floating markets in Bangkok for tourists to visit by themselves is in Taling Chan district (GPS 13.776285 100.456643). Some riverside markets are so large these days that it can be overwhelming for newbie visitors. Taling Chan is more manageable and not so crowded if you go first thing in the morning. You will also find that some of them menus are written in both Thai and English and that there are also bilingual announcements.
I first came here years ago on a long-tailed boat tour from a pier near the Grand Palace.