We have two 300 litre cuves (tanks) for rainwater collection at the orchard. They have lids on, but every now and then one of the resident Common Wall Lizards Podarcis muralis will find a way in. If I don't make sure there is a stick to provide an escape route they can drown. I think what must happen is that they get too cold to manage the climb out, and if the water level is low, it's probably too smooth and vertical. Unfortunately, about once a year, I find a sad little floating body. It's always the juveniles, who I guess have less reserves of energy than the adults. Sometimes they are unlucky, and the wind will have blown the lid off and the safety escape stick out onto the ground.