The best replacement for watir is selenium.

The best alternatives to watir are rated as following:
★★★★★  selenium
★★★★★  capybara
★★★★  watir-webdriver
★★★★  webdriver
★★★  qtp
★★★  selenium webdriver
★★★  coded ui
★★  mechanize
★★  watin
★★  watij
★★  cucumber
★  webrat
★  watir
★  imacros

Your alternative for watir is not listed? Suggest!
Speaking about watir, one probably shouldn't forget about cucumber.

Other search terms related to watir and their ratings are:
★★★★★ cucumber
You have in mind a companion for watir and it is not yet here? Propose!



Google Trends

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watir
selenium
capybara