In NYC, headlines include locals dumping buckets of water on the police officers.
In New York State it is not assault. Third degree assault (the least degree) is defined as:
A person is guilty of assault in the third degree when:
With intent to cause physical injury to another person, he causes such injury to such person or to a third person; or
He recklessly causes physical injury to another person; or
With criminal negligence, he causes physical injury to another person by means of a deadly weapon or a dangerous instrument.
It might be menacing in the third degree, defined as when one
by physical menace, he or she intentionally places or attempts to place another person in fear of death, imminent serious physical injury or physical injury.
The jury would decide based on these instructions, which does not clarify the situation much, but clearly the officer(s) would have to think that there was more to it than just water.
From the new-south-wales Criminal Trial Bench Book (or Judging for Dummies):
An assault is any act — and not a mere omission to act — by which a person intentionally — or recklessly — causes another to apprehend immediate and unlawful violence: R v Burstow; R v Ireland  1 AC 147. Thus it is the fear which is the gist of assault.
Battery is the actual infliction of unlawful force on another. But the word “assault” has come to describe both offences: see DPP v JWH (unrep NSWSC, 17 Oct 1997).
Barwick CJ in The Queen v Phillips (1971) 45 ALJR 467 at 472 described an assault in the common law sense of the word as follows: “Such an assault necessarily involves the apprehension of injury or the instillation of fear or fright. It does not necessarily involve physical contact with the person assaulted: nor is such physical contact, if it occurs, an element of the assault.”
Obviously, being doused with water is unlikely to cause harm, however, the police don’t know its water - it might be battery acid and that lack of knowledge creates the required the “apprehension”.
Its a common law rather than a statute law provision so it should be applicable in all common law jurisdictions that haven't made it a statute definition.