Swim Goggles

Obtained from

 * June 2018 Quest Tap Battle Reward.

Trivia

 * Ranking of units who are equipped with as of 2018/9/30 (tweet):
 * 7,039
 * 6,927
 * 6,137
 * 5,764
 * 4,194
 * 2,944
 * 2,938
 * 2,891
 * 2,865
 * 2,794