Le Coupe d'Aviation Maritime Jacques Schneider (i.e. the Schneider trophy) was a race specifically designed for seaplanes, as they were seen as the only viable long range passenger option in the 1930's.
There wasn't really a comparable land plane race at the time - so the Schneider Trophy race planes improved to the point where they were setting the speed records - and they happened to use floats because the race rules required it.
So you see, it wasn't a case of "Let's go fast - hey we can use floats!" but more of a case of "We have to use floats on this plane, now how fast can we make it go?". Also floats aren't as draggy as you might assume, in flight they produce positive lift for not much more drag than an aerofoil section.