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'
)
)
);