For sandy conditions a 'dry' lube is preferred as it attracts less particulates.
However, for salt air/salt spray conditions a rust-preventative oil or grease is preferred. These tend to attract particulates such as sand that accelerated driveline wear.
The only solution that really works to minimize both wear and corrosion in sandy and salty conditions is frequent cleanings and fresh applications of lubricant. This need not be a dreaded task even with a fleet of bikes to maintain.
With the use of a chain cleaning device (Park Tool, Pedro's, Spin Doctor, Finish Line, etc.), a couple of bike cleaning brushes, a few rags and some lubricant it should take no more than 10 minutes per bike to clean and lube the chains. Frequency will, of course, be based on how often the bikes are used, how long the rides are and under what conditions.