A waterproof Chlorine Sensor
Your best bet would probably be to interface with a colorimeter.  There is a tutorial for an open source colorimeter here:

http://www.appropedia.org/Open-source_colorimeter

IORodeo also sells a shield:

http://www.iorodeo.com/colorimeter

Vernier sells a colorimeter that may be suitable for your project &amp; Sparkfun sells a shield to interface with Vernier sensors.  

If a colorimeter doesn't work you would have to use an electrode, which is pretty expensive and has a shelf life.