Jamie is already soaked, which means the idea of getting wet doesn't bother him much.

On the other hand, Jamie is also lazy. So after unleashing his initial squirt at Mal, instead of running about like everyone else, he holsters his pistol, grabs a bucket, and swings himself up to a convenient perch on a mast.

The water in the bucket is aimed at Inara and Mal. Where it actually ends up is anybody's guess!