He'd probably be pretty miffed. I don't suppose Picard is adverse to the odd prank if it's in good spirits, but being soaked with cold water is a thoroughly unpleasant experience (unless you're at a hydro-spa and do it on purpose, then it's kinda bracing -- but I digress).
We'd have to look at comparable examples, and also consider the changes to Picard's character of the course of seven years. In year one, he was pretty grumpy. Wesley accidentally hit him with a snowball and he was not best pleased. However, since it was an accident, and since Wesley was just a child, he kept his "cool" and let is slide. Picard at that time did not want to lose his sense of dignity. Being humiliated in front of a new crew would not go down well.
Fast-forward seven years and things might be a little different. Picard seemed more easy-going and even participated
in a prank - laughing along with Riker has a newly-promoted Worf got dunked into the holo-ocean. Clearly, Picard can appreciate a well-played prank if it's in the right circumstances.
So, I suppose it depends who is doing the bucket-dunking and in what context. If Riker did it, he might just get away with it, but Picard would have to be having a good day.