I felt that the time limitations on the dives were placed to be fair to all the guests and not for the DM's convenience.
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The schedule is not for the DM's convenience. It is for the convenience of the guests so that they can return and enjoy a relatively long lunch/rest break before the afternoon dive. Most guests I have known over the years have been understanding of this and are quite considerate of the others. But, occasionally, I see some who seem to think that their "station" in life entitles them to set their own rules.