Watir-WebDriver supports the browsers that WebDriver supports.
b = Watir::Browser.new :firefox
Chrome support is through the platform specific ChromeDriver binary, which you download and put on your path.
b = Watir::Browser.new :chrome
Internet Explorer should just work, as long as the protected mode setting is the same for every internet zone (all on, or all off).
b = Watir::Browser.new :ie