Good acid for cleaning marine toilets
A cheaper and easier to find solution than EDTA (permeakra's answer) is citric acid. Citric acid reacts with metal carbonates (bicarbonates, phosphates, chlorides, etc.) to produce the soluble citrates. The citrate ion keeps the metals in solution by chelation, the same process that EDTA (ethylene diamine tetra acetate) does. 

Citric acid can be purchased in the grocery store or convenience store as one of the principle components for any sour powdered drink mix (like Tang).

You could also use any substance or cleaner designed to remove hard water deposits or lime scale, such as CLR, which contains lactic and gluconic acids as chelating agents.