There are two kinds of ballasts; normal ballasts which weigh 150 tons and has a density of 1.2, and heavy ballast which is twice as heavy weighing 300 tons and a density of 2.4. Ballasts also give your ship good stability but if you do not monitor the density your ship won't float. Note that while ballasts are heavy, they only give the same protection as a normal hull block. Ballasts are usually added to the under side of a ship to move its centre of gravity lower in the ship, thus making it more stable.
Tip: Armor blocks can be used when the weight offered by the normal ballast is not enough or the weight needed is not offered by the ballasts. Consider this when designing submarines.