Watercolours are commonly painted onto rich heavyweight and handmade watercolour paper. These papers have very attractive edges which have not been cut. To frame this paper behind a mount would be to reduce its charm.
To float or surface mount is a great option. We would try to reduce any cockling in the paper (caused by water absorption as watercolour is applied) by lightly spraying the back of the paper with water and leaving overnight under a large weight. This prepares the artwork for floating.
All that is left is to choose the right coloured mount on which to float the watercolour and a frame.