Because you often give folk helpful information, I've taken the trouble to have a look through the links on this Thread myself.

These are courses in for the Basic Brevet which are open to anyone who fulfils the requirements. (age etc.)
Although if you've done it before, it will probably be far too easy for you from a practical standpoint, but I doubt if you'll be able to take the SLRG II (which is more likely the standard for a lifeguard) until you've done the first one and I don't know if there is a possibility of doing the theory in English. If you can cope with the German, this page is quite interesting too.

(Things have changed a bit recently, but the methods used for saving people, much to Mr L and my hilarity, used to be quite different from those used at the time in UK!)

If you don't find what you want there, you could get in touch with this guy. He should be able to help.
Lebensrettungsgesellschaft SLRG Region Zürich, Schreiber Daniel
General-Werdmüller-Strasse 18
8804 Au ZH
Tel 044 780 63 47*‎
I googled him so his address is no secret.

I'm not really into the background of SLRG so I cannot speak from personal experience of the way it functions.

Hope this helps though.