Unlike mammals, the glottis (pipe through which they breathe) in snakes and terrapins is extended way out into the bottom of their mouth. It allows them to breathe even while swallowing. This also means that theres nothing to stop water from just spilling out of their stomach by way of gravity. This normally isn't a problem when laying flat but if the upper third of their body is lowest, gravity makes the water just run right out of their stomach. There's nothing to stop it. Here you can see the glottis. It opens and closes as they breathe through it. It leads to the lung. Everything above that is a direct line to the stomach with nothing in between to stop water from just flowing right out.