Not a 22 pilot myself.
Mariner
F-GTCK - Private Robinson R22 Mariner series Aircraft Photo | Airplane-Pictures.net is floated but I think that there are significant differences between the mariner version and the non. I think that in construction the anti-corrosion treatment is different and this may be required for certification for use on water.
Not aware of any after Market pop-out floats available for the 22.
Have you asked Robinson?