Hi Daniel,

I asked myself the same thing during this summer and didn't go through as the weather wasn't great and not really the time for it but here is what I understood and found at that time.

it depends of how often you are going to fish...

No license needed if you fish with just one line and a single hook.

You may get a daily license for 10.- in some shops. I did buy it once at fish shop "Chez Nestor" in Eaux-vives,

or you can get a yearly license, cost approx 140.- but as I undersdtood you need to pass some kind of seminar given by authorities in order to get it. 

you can get all the details for Geneva state here

http://ge.ch/nature/peche-infos-et-permis/peche-au-lac

cheers

Aymeric