Meaning:
Literally this term refers to the action of rowing a boat with oars. In ancient times when there were no fuel-driven ships, many sailors had to row a big boat together. While there were always a few people who wanted to slack and save some strength, these people just pretended to be rowing by slapping their oars onto the surface of water. Therefore, the term is a metaphor for slacking at work or during other group activities.
Example:
A: 昨天的合唱表演小花一直在划水。
Zuótiān de héchàng biǎoyǎn xiǎohuā yīzhí zài huáshuǐ。
Xiao Hua was lip-syncing during yesterday’s choir performance.
B: 她排练的时候很认真啊，可能是碰巧嗓子不舒服唱不了。
Tā páiliàn de shíhòu hěn rènzhēn a，kěnéng shì pèngqiǎo sǎngzi bù shūfu chàngbuliǎo。
She put in a lot of work during rehearsals. Perhaps she had a sore throat and couldn’t sing yesterday.