Now this is a unique way to really get around Bangkok. Best described as being dirty and dangerous... but also handy, vibrant and exhilarating.
There are two options, the klorng boats, which run from 5:30am to 10:30pm, with ticket prices around 8B to 20B. These hold about 40-50 passengers, have dubious safety equipment, unreliable timetables but zoom down the canal at breakneck speeds. Whilst they have plastic tarpaulins you are fairly low to the water and always face the possibility of getting wet from the spray.
The other alternative is the tourist hop on/hop off boats which run from Pratunam (bear the Central World Plaza) and Panfa Leelard, just behind the Mahakan Fort, at Phanfa Bridge. These are more expensive, but cheaper and quicker than a taxi, they give you the same experience as the local klorng boats but at a more leisurely and safer speed. Can purchase a one-way or all-day ticket.