Really ? Surely it would have to be unbelievably deep for one to drown wouldn't it or have steeply sloping inwards sides !??
They do have decent muscle tone , clearly not as much as a Boa but I've seen them telescoping around and stretching up whilst yawning . I actually think , as mentioned above , that it's possibly just a case of wasted space as they probably won't spend long soaking like many other types of snake do .
Just in case, you know LOL. Since tubs usually do have vertical sloping sides, so it got me thinking the BP could drop in unexpectedly. In the natural world, water is first shallow on the edges and all.