The best replacement for water injection is intercooler.

The best alternatives to water injection are rated as following:
★★★★★  intercooler
★★  nitrous
★★  gas injection
★  methanol injection
★  hho
  intercooling
  propane injection
  steam injection
  intercooler spray

Your alternative for water injection is not listed? Suggest!
Speaking about water injection, one probably shouldn't forget about nitrous.

Other search terms related to water injection and their ratings are:
★★★★★ nitrous
★★ e85
You have in mind a companion for water injection and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of water injection in all years. Click on graph for more.
water injection
intercooler
nitrous