How do I get the form submitted data in a custom module?
You have to get submitted data based on sid, I think below code will be helpful for you 

 function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id){
  if ($form_id == 'WEBFORM_'){ 
module_load_include('inc','webform','includes/webform.submissions');
$sid = 10;
$submission = webform_get_submissions(array('sid' =&gt; $sid));
$nid = $submission[$sid]-&gt;nid;

$web_submission = webform_get_submission($nid, $sid);
$node = node_load($nid);
$output = webform_submission_render($node, $web_submission, NULL, 'html');

print $output;
}