Peel and cut the potatoes into 1-inch (2.5 cm) cubes.
Put the potatoes and water in a medium saucepan, cover, and cook over medium heat for about 10 minutes, or until tender. Add the drained tuna, sugar, soy sauce, and sake. Cook and stir, uncovered, for 10 minutes. Add the peas and cook for a couple minutes more. Cover and let cool.