Aside from the toilet-esque connotations this word may have for the post-Al Bundy American public, it also has a frequent use in the North American freight graffiti 'scene'.
The term floater refers to a piece on a freight car that does not extend to the very bottom of the car. From about 1999-2000 there seemed to be a sort of negative stigma attached to the 'floater' for a lot of freight purists because one of the main motivations behind not going right to the bottom seemed to be "fear of the bar".
That is to say: floaters were/are (?) looked down upon because they seem like a cop out. Rather than going all the way down to the bottom, and dealing with the (sometimes) difficult obstacle that the 'bar' on a boxcar presents, unskilled or lazy writers avoided the bar altogether.
But, of course, this opinion is not that widespread, and is not universally applicable in any case. For instance: at many factory layups, people are forced to paint floaters because there is a loading dock covering the lower half of the train. In addition to this pragmatic concern there are also stylistic reasons behind the 'floater'. For instance, one might want to integrate one's piece with a particular section of the car one is painting (the company logo, etc.).
The idea that a 'floater' is inherently evidence of a poor writer is a flawed one, and it is easy to spot the difference between someone who floats because they have to and someone who floats because they chose to.