When something toxic gets into the eye, flushing with large amounts of room-temperature liquids can reduce injury to the cornea and help systemic absorption, if that is an issue.
Normal saline is preferred for use in the eyes. However, with the duration that most eyes must be flushed, you may run out of sterile eye irrigation solution. In those cases, room-temperature milk is a good second choice.
Tepid tap water is ok to use in a pinch, but not the best solution. For most liquids, 10 minutes of flushing is ideal. However, for caustic substances, 15-20 minutes of flushing is absolutely essential.