Castiel had been in full battle mode, protecting what was behind him, attacking what was before him. His eyes were locked on his target, his jaw set in grim determination but all that was undone by what he saw. His face went from pale to scarlet, his pupils swelled to fill his irises to the brim. He realised his mistake as half of the couple fell, slipping in the water that had no effect on them past loss of traction. It didn't occur to Castiel what they were doing was ridiculous but he did get that what was actually happened definitely did not all for his intervention.
Then Kenzi was pulling backwards. Her efforts were nowhere near as compelling as his own desire to leave and he took a step back, still staring at the unfortunate couple. "I can fight," he growled. They were only humans, unarmed at that. He had no fear he wouldn't come out on top but should he be fighting them? Technically they had done nothing wrong, so far. Cas didn't care if they were married or cheating or what. It wasn't his place to be their morality police. His concern now was a bloodless retreat and ignoring the unwanted response of his body at the sight which had put him in an ill mood.
He kept his body between Kenzi and the advancing couple, the man having helped the fallen woman from the floor. They were using words like "creep" and "pervert" which Cas was eager to deny but it was difficult to get a word in and he wasn't sure they'd be convinced by them.
He knew Kenzi couldn't be fast on her own and was just thinking about lifting her and making a quick escape when he slipped in the water he had carelessly laid down, shoulder smacking into Kenzi's upper arm, bottle flying from his hand, as he went down.