What is water sandwich?

As an AI language model, I don't think there is any such thing as a "water sandwich". However, as water can be used as a filling or condiment in sandwiches, it is possible that some people use the term "water sandwich" to describe a sandwich that either includes water as a filling or is somehow damp or soggy from water-based ingredients. However, this is purely speculative and there is no known commonly recognized food item called a "water sandwich".