Lithium reacts intensely with water, forming lithium hydroxide and highly flammable hydrogen. The colourless solution is highly alkalic. The exothermal reactions lasts longer than the reaction of sodium and water.
Notice that fishes and other aquatic species can not survive in lithium hydroxide. I think that all creature in solution in the same side as the golem should also die.
Hmm... perhaps I could change the name to something else in that family? What does potassium do, and is there anything special about sodium's reaction with water? (I'm asking you because I'm pretty clueless about chemistry. Thanks for the help!)
Actually, all three alkali metals (Sodium = Na, Lithium = Li and Potassium = K) react in the same way. In contact with water, they all form hydroxide (OH- together with the corresponding cation Na+, K+ or Li+), hydrogen and a lot of heat. The only difference is the "speed" of said reaction and the amount of energy (heat) created. In some cases, the hydrogen gas may explode.
Concerning what is left in the water, I'll only talk about the most common one Sodium hydroxide (the two others have similar behaviour). Sodium hydroxide (NaOH), also known as lye and caustic soda, is a caustic metallic base. Solid sodium hydroxide or solutions of sodium hydroxide may cause chemical burns, permanent injury or scarring if it contacts unprotected human, or other animal, tissue. It may cause blindness if it contacts with the eye. Protective equipment such as rubber gloves, safety clothing and eye protection should always be used when handling the material or its solutions. Dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables. It also produces heat when reacted with acids. Sodium hydroxide is corrosive to some metals, e.g. aluminum, which produces flammable hydrogen gas on contact. Sodium hydroxide is also mildly corrosive to glass, which can cause damage to glazing or freezing of ground glass joints.
Your idea makes sense because something made of sodium will explode (or something close to this) when put in contact with water. However, as I told you the water itself would not be left intact. To reflect this, I think that a mechanism similar to unstable gas could be implemented (every other creature in the water takes one/two damage).
