It's by chance... I've had snakes drag it into the water and then "lose" the prey when they let go, because they forgot they left it in the bowl. And then I have to reheat and reoffer. They're not very bright animals when in feeding mode LOL.
So now whenever I see them get close to dragging it in water, I use tongs and put them further away (while the snake is wrapped around it), or I take the water bowl out altogether before offering food.