Hard water contains high concentrations of dissolved minerals, primarily calcium and magnesium ions. These ions react with soap to form an insoluble scum, making it difficult to form lather and clean effectively (Statement II is correct). In boilers and heating systems, these minerals can precipitate out upon heating, forming scale (limescale) which reduces efficiency, obstructs flow, and can cause damage (Statement III is correct). Hard water is generally safe for drinking (Statement I is incorrect, although excessive hardness might contribute to issues in some individuals or systems, it's not universally unsuitable). The suitability for irrigating crops (Statement IV) depends on the specific mineral composition, soil type, and crop tolerance; while highly saline or sodic hard water can be problematic, not all hard water is unsuitable for irrigation. Therefore, washing clothes with soap and use in boilers are the most consistently cited instances where hard water is unsuitable.