a flask or water. Certainly it wasn't a bottle.
To an American like Peter, a soldier's container for carrying water is called a canteen -- here's an antique example from the US Civil War:



But the word "flask" (for an American) would suggest a small, easy-to-hide container that a soldier uses (generally against the rules) to carry vodka, whisky, gin, etc. -- Peter could count himself lucky indeed if it turned out that the soldier had a flask of alcohol, in addition to his standard canteen of water!