You don't need to hold an instructor rating to teach floats, just 50 hrs seaplane. That being said if your students are PPL, they will not be able to count the dual time with you as dual for the issuance of the CPL
Canadian Aviation Regulations - Part IV, Standard 425 - Flight Training
If you hold an instructor rating AND the FTU you are attached to is licensed to do seaplane training I BELIEVE that forgoes the 50hr requirement... but don't quote me on that... not to mention its probably a piss poor idea...