I looked for the same a year ago. Being unsuccessful, I enrolled my (then) four year old in Knechtli Schwimmschule. I have been happy with the choice. There are many locations and times available for the various levels and you can book online. At the time my child did not speak german but for the most part she just followed along with what the other kids were doing and all was fine. When needed, the instructor speaks English to her but it is hardly necessary as he teaches mostly by example. I have also, when my German was not sufficient, spoken English with the administration, no problem.
Here is their site: http://knechtli.kursorganizer.com/CMS.aspx?textid=12