To put his into a new post as a thought... shock.

Reptiles, especially snakes do not respond to rapid temperature changes well. Ball pythons can withstand pretty severe temperature gradients so long as they are raised or lowered gradually. A sudden rapid change in temperature such as a body temp of 70 something degrees from a cooler tank quickly dumped into what feels like luke warm water but might be 90+ degrees is a rapid shock to the system. Now I am not being any expert here but just speculating due to the statement of a snake thrashing in water.

This snake could already have had a weakened immune system and if put into a rapid temperature change could have gone into shock. The fatality of it might not happen right away but it could lead to the snake's demise. This might not show in a basic necropsy because it would require enzyme and blood chemistry that is already difficult to get in a live snake, let alone a deceased one.

It is crucially important to know the actual temperature of not only your snake's enclosure but your snake. Unexpected and sudden temperature spikes can be fatal to a snake, especially one that might be weak from illness or stress to begin with.