You might got things confused. Usefulness is a different from value. A necklace might have big value both monetary and sentimental but it has very limited usefulness. A knife however is very useful though it is very rare that one would be of great value to someone. Also your example: water is very useful. Cooking, cleaning, bathing, cooling, paper making, dyeing, etc. makes it one of the most useful substance, whether you have a lake or only a canteen of it. Its scarsity or availability doesn’t change its usefulness.