you wrote a park is a karfaf and prohibited from carrying
most parks have water fountains
once there is drinking water it should not be a karfaf – just like pasei biraos for olei regalim [rashi beg. of perek osin pasin]
I am not sure if the water fountain alone would make it “hukaf ledirah”, but might be considered hukar ledirah because it provides many uses for the general public.