Seriously, the Dursleys would have not only let her drown, they would have let her drown so they could blame Harry for it afterwards. (Although the loudest “Potter, too busy winning to care about anyone else” voice in his head would probably be Snape’s.)
Incidentally this is even more clear in the first and second books, to me. Because Harry DID go to adults and say someone’s trying to steal the stone, and what did the adults do? Did they say, yes, we know, we’re taking precautions, real, good protective measures? Noooo. Did they say, thank you, we’ll look into it, even? Noooo. They said, don’t be silly, it’s not your concern, nothing to see here, little boy, run along and do your schoolwork.
And they said this to a boy whose entire life experience has never involved an adult that can be depended on. And they lied, lied about their own knowledge, said “that’s silly” when they know “that’s true.” And they were too convincing: since he as well knew the truth, what they ended up convincing him was that they didn’t know. And it fit right in with his expectations. Adults, whether actively malicious (the Dursleys, Snape) or well-meaning but oblivious (Mrs. Figg, Harry’s primary-school teachers, the other Hogwarts teachers), can’t be depended on. If anything’s got to be done, Harry and his friends have got to do it himself.
Second book, same thing—they’re headed for the teacher’s lounge to tell the teachers it’s a basilisk, and overhear the teachers saying that Ginny Weasley’s been taken by the monster, and they need to close Hogwarts, and their only plan to rescue Ginny is to send Gilderoy Lockhart—knowing full well he’s a fraud, a coward, and no match for a Cornish pixie, let alone a basilisk. Once again, the adults are flat-out useless and if anyone is going to save Ginny, it’s gotta be Harry and Ron.
Notably, this is after another ball-drop on the part of the adults: when Harry’s been framed for underage magic and locked up in his room and starved by people who have every intention of keeping him out of Hogwarts forever, it’s other kids, Ron, Fred, and George, who go rescue him, and when the adults find out, one of them punishes and scolds and the other is only interested in how his car worked.
In book three, we meet a couple of adults that are competent, helpful, and willing to listen—Sirius and Remus—and the other adults come in and the end result is, one’s fired and the other has to go on the run lest he have his soul sucked out by dementors. Dumbledore does listen and give them the necessary hints, but it’s Harry, and Hermione this time, who have to do the work.
And then in Order of the Phoenix, in comes the smothering bullshit about how he’s too young to be in the Order and needs to leave everything to the grownups, after the grownups have dropped the ball four years running and are batting zero on the trust-and-listening factor—no wonder he threw a tantrum, I would’ve thrown a tantrum, he was fucking entitled to one.
