I recently took a local ferry as part of a work commute in Bangkok. I loved it! It cost 20 baht for one version and 15 for the other (not sure the difference) and it takes no time to go up and down the length of the city, and you're on a boat. It looked as if there were different versions that stop at different places, like a local or express train version, but I basically just got on the boats without noticing much about that or signs that would have said anything.
River dinner cruises are related but a quite different thing, where they just go up and down to serve as a floating restaurant, more the kind of thing you set up and reserve for a group.
Does anyone have input about other boat travel in the canals? I've seen such a thing but I'm not sure how it works; surely there must be a limited range of where that would run. With the traffic as it is in Bangkok it would be nice if they had more coverage but if someone is going up and down the Chaophya river that's easy enough. The bus system is a bit complicated here so I guess the catch is that unless both the start and end are by the river (or canal) there's still a local travel hurdle involved, but the BTS does run from a couple bridge / pier stops too.