The only critters I've been worried about falling through a solar cover and drowning are my own dogs - we have three and none of them are comfortable in water. They do run across the safety cover in the winter, so I guess I"m worried they'll mistakenly believe the solar cover will hold them. I've been hesitant to get one for this reason, though I did recently take the plunge and finally buy one. We have a heat pump and the expense of heating the pool during the day, only to lose half of our gains at night can get expensive over the course of the summer.
I'm still worried about it, but we'll just have to be extra vigilant when the dogs are running around the back yard. As for smaller critters, the aforementioned dogs see to it that nothing gets near our pool (or in our yard) to begin with! haha