Concrete Donkey's unused water image
It appears the Concrete Donkey (like all weapons) has an underwater sprite for when it hits the water. However, it's not actually used - it just sticks with the standard sprite for both above and below water level.
Maybe its a bug or perhaps an oversight by Team17?
Either way, could this sprite be used?
AdminDeadcode (Admin, Worms Armageddon) commented
It wouldn't make sense to use the underwater sprite unless the Concrete Donkey splashed and sank more slowly once it hit water.
The Donkey is so huge that its top half disappears suddenly once goes below the bottom of the ocean and is deleted. It is deleted "too soon"; with slow sinking this would become much more noticeable. I think this might be why Team17 made it fall off the screen instead of being affected by water.
Having said that, it's possible to make an exception for the Donkey and require that it sinks further below the ocean bottom, so that its sprite can scroll completely out of sight before being deleted.