Yes, as long as it is a complete coating it works as a kind of paint, if it is pierced it begins to work as a sacrificial anode, at least at this area.
As far as I know zinc carbonate is soluble in fresh water only in very small amounts, but I don't know about sea water. I think this depends on the kind and amount of other ions (esp. chlorine) present, and as one can deduce from the fact that sacrificial anodes work, it will be destroyed when the zinc is connected electrically to a chemical nobler metal.
Don't know about Zincalum.