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I have some background in geotecnical engineering, and in general if you are constructing with sands you want 1 part water between 8 to 10 parts sand. It varies a lot depending on the amount of fines, the more fines the more water it can hold and the more you can sculpt it. Fines being silts or clays mixed with the sand particles.
As an engineer, the answer could get a lot more detailed. I could go into adding strip reinforcement with grass from dunes...estimating the friction angle from dry sand...running a four point optimum moisture content test....but, for most people that would take the fun out of it. I, however, would enjoy building the strongest, longest lasting, sand castle...so I would do all the estimations...on top of estimating the gradation of the granular particles. The best sandcastle sand would be angular particles evenly distributed in size (small, medium, and through large particles). Also, if possible, you would want to compact the sand in layers during construction.
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