The 'Safe drinking water' part of the title of this legislation is a bit of a misnomer; the legislation refers to exposure to the listed chemicals by any means including consuming them in food.
I work in spices and P65 affects us in several ways- there are P65 chemicals that are natural constituents of spice (e.g. styrene in cinnamon), certain fumigants used in treating spices are on the P65 list, primary packaging may contain P65 chemicals (although many manufacturers have removed them), P65 chemicals may be a natural contaminant (e.g. lead in soil being taken up by plants) and byproducts of ethylene oxide treatment (epichlorohydrin) are on the list. The P65 list linked above by Mohamed Yusuf has the chemicals and the exposure level at which a P65 warning label is required. You will have to decide for yourselves if any P65 chemical is present in your product at a level where consumption might require a warning.
Although this legislation is relevant to products sold in California as Scampi states, you should be prepared for this if you sell anything into the US as your product has the potential to be sold throughout the country. As the law is changing slightly from August (basically any warning on the pack must now call out the specific chemical and whether it causes reproductive harm or cancer) there is renewed focus on P65 at the moment and we are finding more and more customers looking for statements on P65. If you have US customers I am sure these requests will be coming!
One of our group companies has already faced a P65 ambulance chasing class action lawsuit- not sure how that might affect a Canadian company but it's something that should be considered.
Finally, we are struggling with how to write a P65 statement that covers our legal bases currently so there is no easy answer- this legislation is a pain to say the least!