Mix the water with cucumber slices. Refrigerate until ready to serve.
In a small mixing bowl break up the tuna with a fork. Toss with celery, onion, lemon juice, and mayonnaise as needed until the tuna is to your desired creaminess. Season with salt and pepper to taste. Stir to combine.
Lightly toast the bread.
Assemble the sandwich: Layer the bread with lettuce, tomato, and tuna salad. Top with a slice of bread then cut in half.
Serve: Enjoy the sandwich with chips, pickle, and cucumber water.