Brass is frequently used in marine environments because of its extreme resistance to corrosion. For this reason brass artifacts are typically in excellent condition even after hundreds of years. Brass is an alloy of copper and zinc and is extremely heavy. It is prized by divers fro some reason yet unbeknownst to me. I think it has to do with the durability of the material and the excellent condition of the artifacts found. Because brass is so durable it cleans up rather well with a great deal of patience.
The first and most important thing to do is soak the artifact in fresh water. The longer the better. The salt and minerals found in ocean water work their way into the porous metal and will later cause havoc if not dealt with properly. One way to do this is to utilize a submersible pump available from most pet stores or aquarium supply stores. Place the pump and the artifact in a bucket or container large enough to submerge both objects. Connect a piece of tubing to a faucet and let it run into the container. Connect a second piece of tubing to the pump's output and route it to some drain or onto open ground. Fill the bucket and start the pump. With a bit of patience you can adjust the input of water to match the output of the pump. This provides the artifact with a constant flow of fresh water. It works well for larger objects. A more popular and economic method which works well for smaller objects is to place the object in a toilet tank. Each time the unit is flushed the tank will refill with fresh water.
Once you have thoroughly rinsed the object, many people feel that the easiest way to remove encrustation from brass is to use an acid wash. Muriatic acid which is easily available from pool supply stores is the easiest to work with and obtain. It is also preferable to something less caustic such as acetic acid (vinegar) because Muriatic acid is a weak form of hydrochloric acid (HCl) which breaks down into its hydrogen and chlorine atoms, both of which are already in water and therefore theoretically will not cause any extra contamination or unwanted reactions. The acid wash should be kept to as short a period as you can get away with. After the acid bath, it is imperative that the object be soaked in a mixture of water and baking soda (sodium bicarbonate, NaHCO3) this will neutralize the muriatic acid and stop any corrosion to the metal that extended contact may cause.
In place of an acid bath, nothing beats good old elbow grease and a brass bristle brush. Scrub hard, soak, scrub some more, tap lightly. With a lot of effort and even more patience the item will clean up nicely. Frequently a dremel or other such rotary tool can be used with either a brass brush or a rubber polishing wheel. Be aware, these tools may leave marks on the metal as they may take a top layer off. By far the most benign treatment is time, patience, elbow grease and lots of soaking in freshwater.
A final polish with some buffing compound.Once the item has been thoroughly cleaned can give it that like new shine. You may wan to seal the item with a clear lacquer or acrylic to offer protection, but realize the item must be dry before, any minerals trapped inside may work their way out and cause a reaction under the topcoat.