In the movie Suicide Squad why does Batman kiss Harley Quinn after rescuing her from the water?
As per Rogue Jedi's explanation, he was performing mouth-to-mouth resuscitation. I believe that he first checked that she wasn't breathing and/or had an absent pulse. She then turned it into a kiss as per the trope.
On a side note, what he did was dumb. If you don't have a proper protective barrier (And no, rubber lips would not count), even if trained, it is recommended to do hands-only CPR. Otherwise, you're putting yourself at a high risk for diseases and biting incidents, particularly since vomit and mild psychosis are common side effects of resuscitation. When doing it on a known violent person such as Harley... incredibly bad idea.
And I found a (poor quality) bootleg of the scene. He does check her pulse. His movement is deliberate and fast once he decides to give her mouth-to-mouth, but really, that describes everything Batman does.
According to the film's official novelisation, he wasn't kissing her, he was performing CPR after having determined that she wasn't breathing and that her pulse had stopped.
Interestingly, the scene lasts very slightly longer in the book, making his actions rather more comprehensible.
Gripping her firmly, he swam back to the surface. Reaching land, he lay her on her back and administered chest compressions. No response. Only one more thing to try. It was repulsive, but her only hope. Batman put his mouth to hers, this woman who had just tried to kill him.
He alternated breaths with more chest compressions.
Suddenly she wrapped her arms around him, turning CPR into a prolonged kiss. He fought and pulled away.