* Sodium is highly reactive: Sodium is a highly reactive alkali metal that readily reacts with water, producing a lot of heat and hydrogen gas. If a ship were made of sodium, it would react violently with the water it's floating on, creating a potential explosion hazard.

* Sodium is soft and weak: Sodium is a very soft metal and would easily deform under pressure or impact, making it unsuitable for the structural integrity of a ship.

* Steel is strong and durable: Steel is a strong and durable alloy of iron and carbon that can withstand the stresses of the ocean environment. It is also relatively inexpensive and readily available.

* Steel is resistant to corrosion: While steel can corrode, it can be coated with protective layers like paint or galvanizing to prevent rusting. Sodium, on the other hand, reacts with water to form sodium hydroxide, which is corrosive to most materials.

In short, sodium is simply too reactive, soft, and unstable to be used in shipbuilding. Steel, on the other hand, offers the necessary strength, durability, and resistance to corrosion for a safe and reliable vessel.