An addition to a double boiler to expand its use is a steamer. I personally use a stainless steel double boiler which has a steamer insert with handle.
On board a boat the above three piece arrangement works well if your galley has a two burner stove. For instance, for dinner potatoes can be boiled in the bottom, and your favorite vegetable steamed in the steamer. The top of the boiler can be used for warming rolls, sauces, or heating an already prepared side dish.
The steamer can double as a colander eliminating the need for a separate strainer on board.