It's ironic that Fred should want water at the start of the story as he'll soon volunteer to risk life and limb to fetch water for his comrades. In a classic example of situational irony, the opposite of what we'd expect to happen actually occurs. Though he's clearly rather thirsty, we wouldn't expect Fred to go to the extraordinary lengths he goes to in order to fetch water. Skipping back and forth between the lines as the bullets fly all around his ears seems like a supremely reckless act on Fred's part. And for what? A single bucket of water.
At the start, it seems that Fred wants water for himself; he's not really thinking of fetching water for the men in his company. And yet that's precisely what he attempts to do, risking his neck in the process. But in yet another irony, it turns out that his heroic efforts are all for nothing. Instead of cherishing the bucket of water that Fred brought back to them at considerable risk to his own life, a couple of foolish lieutenants treat it like it's all a big joke and end up spilling the water. As this is a wholly unexpected outcome, it's another example of situational irony.