The great white's knowledge and sensory system lead it to that particular site on the prowl for its favourite food - weak inexperienced, vulnerable and isolated seals it can stalk and pick off with minimal fuss and potential danger to itself. It did the stalking and had an eye on one or two but instead of an isolated seal emerging as they had all fled having sensed the shark in the waters, there was another alien object in the waters, an object that gave off all the right signals of prey if not exactly usual prey to this hungry shark. Vulnerable, weak, large enough, with a pulse and struggling Shirley Anne had triggered a predatory response from the shark which did what it does best and launched a ferocious attack on her. The shark didn't mistake her for a seal or any other creature for that matter however, it may not have known exactly what she was, but she represented food on that particular occasion all the same, nothing more.
After the initial strike, the shark may not have liked what it had bitten into but that doesn't add any weight to MI, I've seen footage of these sharks attacking a real seal only to spit it out and disappear for whatever reason. Same with many other objects, inanimate, alive, small and large.
All attacks on humans are triggered by predatory responses, curiosity and investigation purposes or maybe even acts of dominance and aggression. Mistaken Identity is a typical human response to explain the unimaginable and that's a dangerous theory to push to a world that has almost zero understanding of sharks and their behaviours. Eric Ritter was in murky waters, there had been chumming, bait was all around him - perfect conditions for a MI attack, was the bull that attacked him mistaken however? You've seen the footage...
By the way in the animal kingdom some of the dumbest creatures are the most successful because they are equipped with the right tools for their environment, if not the smartest of brains. The great white in particular is arguably the most equipped of any animal on this planet for its environment, after all, it isn't the oldest living creature or one of them for nothing...

