Except that doesn't appear to be the case here according to the initial judgement. This wasn't an autistic person causing harm to others by his actions and thus requiring an immediate response. It is perfectly possible to get the public out of the way of the individual (i.e. out of the pool) and then take time to sort things out.
That gives you time to do things like get an accurate account of what is going on, think to yourself "I wonder if they have a carer here?" and decide if you do feel compelled to go into the pool what the most appropriate way to do so is (perhaps not looking like a cop...)
If the individual is in the pool and no members of the public are present then the only risk is to themselves, and I'd be quite happy leaving that to the lifeguards in terms of what to do if they start drowning/getting into difficulty. Them being actually trained in how to get panicking people out of the water.
At lot of this comes back to the age old question - "if it were my autistic child in the pool how I would I want the police to react?"
It's an autistic person that can or cannot be reasoned with for an indefinite period of time? An hour? two hours? ten hours? Two units and a supervisor tied up for how long?
If their is no risk of harm, why are we even there? There are carers in place, obviously enough time has passed that one of them can leave and meet dad at the school.
As Fry says, on this basis, police should say, we've attended on the basis of the call, but the talking down can continue to be done with those who understand his needs the best and leave.