Rustoleum makes a paint for plastic lawn chairs (made of polyethylene). It will work on kayaks. Available at home despot. However, I think it will still chip over time. Design your paint scheme to go well with the chipping. A camo scheme perharps?
/bing
primer
http://www.rustoleum.com/CBGProduct.asp?pid=119
paint
http://www.rustoleum.com/CBGProduct.asp?pid=118