I think he felt bad for dumping the water out in defiance when he saw Kate so tired and thristy and worn down...
and it pissed him off, so he did the one thing that would make him and Kate feel better, he kissed her passionately, to let her know she was important to him and that he cared... he also did it defiantly...
spilled water is one thing but try is on... you scummy others...
I was just surprised that Kate didn't get into the action, she's usually more of a fighter... she is getting done in, run down and Sawyer if seeing and feeling it for her...