What an awful tragedy - I'm so sorry for the family. I think in these sorts of places you really do need one-on-one adult supervision for u-5s and weak swimmers. I have a toddler and a weak swimmer, and the only pools I will take them to on my own are 'boring' ones with a lifeguard where I can see the whole pool from one spot. And of course bright orange/red/pink swimsuits. My younger one stays within arm's length at all times and my eyes follow the older one around the pool.
We visited this place last year with friends, and even with an adult per child it was still very hectic and busy. I lost sight of my older one a few times for a few seconds, but he had instructions not to go in the water or down a slide unless with one of us.