What you have is a pond.
The doctrine of attractive nuisances has state-specific nuances, so you should consult a lawyer in your state to see what the local facts are. The question of liability only arises when there is some harm done and the court either determines that you are responsible (liable), or not. Until there is harm, there is only a risk. In general, you would be held liable if a child came on to your property, got in the pool, and drown, even if you had signs saying "Don't get in the pond". The general rule comes from R.R. Co. v. Stout, 84 U.S. 657, that in dealing with hazards that would be attractive to a child of "tender" years, you have a duty to exercise reasonable care to protect them against the dangers of the attraction.
What constitutes "reasonable care" is hard to define, except in the most tautological sense "the care that a reasonable person would have". Case law in your state may specifically address the question. As a general rule, you should get legal advice from lawyers, not from city engineers. The doctrine may be spelled out as more specific rule such as in Washington (Schneider v. Seattle, 24 Wn. App. 251, the requirements are
that a condition or instrumentality existed which was dangerous in
itself and likely to or probably would cause injury to those coming
into contact with it;
that it was attractive or enticing to young children;
that plaintiff was incapable, by reason of his age, of comprehending
the danger involved;
that it was left unguarded and exposed at a place where children of
tender years were accustomed to resort, for play or amusement or
gratification of youthful curiosity; and
that it was reasonably practicable and feasible either to prevent
access by children or else to render it innocuous without obstructing
any reasonable purpose or use for which it was intended.
The record does not indicate the details of the injury, except that the area was in fact fenced with a cyclone fence. There may have been a gap in the fence, or the child (who was 10) may simply have climbed the fence (the latter seems likely since the ruling also invokes questions of contributory negligence – "even if the child is held to be within the attractive nuisance doctrine, the question of his contributory negligence, in the light of all relevant circumstances, will have to be decided"). FYI the final outcome was that the plaintiff lost the case.
A gate is less of a precaution against trespass than an impenetrable fence, especially if the fence is solid wood and makes it impossible for the child to see the attractive nuisance. A locked gate a more of a precaution than an unlocked gate. Your reference to illegal means of entry is a bit misguided, because the doctrine say that adult trespassing standard don't apply to children, therefore you cannot assume that the child will always act in compliance with the law. A latch at the top of a 6 ft. tall gate cannot be defeated by a "tender" child wandering in a curious manner (unlike a latch at 3 ft.).
As I say, you have to research the specific liability states and case law of your state.
