If you go to Bangkok and stay mainly on the shopping strip of Soi Sukhumvit, you may not even get a chance to see the Chao Phraya River. But the River is a big part of the city life, and it is still the main form of transport for many of the 9 million people living in and around Bangkok.
Besides the MRT which has made getting around Bangkok such a breeze, the well-developed water taxis is another convenient way of getting around. In fact for some destinations such as the Old City (Rattanakosin) the water taxi is more convenient then the MRT, as the nearest MRT station is quite a distance away - whereas the water taxi brings you right to the door step.
The other bonus of using the water taxis is you get to experience the Chao Phraya River, see what life is like for the locals, and soak in the beautiful views along the way.