There is a cookie API in Watir-WebDriver 0.5.2 onwards, that is easy to use.

require 'watir-webdriver'
browser = Watir::Browser.new
browser.cookies.clear
browser.cookies.add 'foo', 'bar', :path => "/", :expires => 10.days.from_now, :secure => true
browser.cookies.delete 'foo'
browser.cookies.to_hash