I'm getting a lot of flack on here, but I haven't said it was child abuse. Ironic that the people saying others are getting on their high horse and crying abuse are... getting on their high horse to overexaggerate what people who don't condone the OP's actions are saying. Fucking Pegasus is needed here! 
Saying something was crappy parenting in that moment is not the same as crying abuse. Same as there are not only 2 options to chivvy a kid into getting ready: smacking or throwing water over them! Some people are worryingly unable to see or understand nuances and insistent on making 2+2=5 (I dunno if I'm being too ancient with that particular saying).
I am impatient and a right bad-tempered old cow, but I wouldn't throw water on or smack a coworker who was pissing me about making me late to go home, or do that to some twat farting about in front of me at the checkout, or to DH if he was driving me nuts. So I especially wouldn't do it to someone more vulnerable than me, dependent on me and who can only take a negative message from it.
Incidents like this are bad (not hugely, I must add for the 'if you suggest anything negative you are crying abuse' brigade) because they do not happen in isolation. You do this sort of thing once, you've given yourself permission to lower your standards and get that little kick that you won't admit to from the power you felt in the moment of throwing that water. And so you end up doing a series of minor crappy things that culminate in an overall picture of pretty across-board crappy parenting. It's the big picture, the long game. Parenting is not about getting what you want from the child in that specific, isolated moment no matter what it takes, it is all about the bigger picture and the long game. Things like chucking water or smacking do not work not only because they are cruel (again, not cruel like Rose West cruel. Sigh) and because you can't expect to change bad behaviour by being badly behaved yourself, it's because they do not teach the child how to behave better, they do not offer the child solutions about making better choices for themself. Oh yes, they might behave better after a while, out of a sullen, resentful wariness to avoid undesirable treatment, but that's not genuine good behaviour and it can affect how they treat others and react themselves, not just as kids, but for the rest of their lives. Or they might just behave better eventually because they matured and no thanks to the smacking or water throwing.
I'm not saying this single incident will and can do all that, I'm saying that repeated incidents of this ilk can do. That's been my point all along.