Only display post if published in last 24 hours?

As birgire said date query is an array of arrays
May be this solves your problemo.

$args = array(
  'post_type' => 'surf_reports',
  'posts_per_page' => '1',
  'category_name' => $cat (this is pulled dynamically in my template),
  'date_query' => array(
     array(
           'after' => '24 hours ago'
           )
     )
  );