The right code to use would be something like:
<option><?php echo((isset($_GET["invalid"])?ValidatedField("boatersupdate","endorsement_sail"):"".$row_WADAboaters["endorsement_sail"]."")); ?></option>
<option value="no" <?php echo((isset($_GET["invalid"])?ValidatedField("boatersupdate","endorsement_sail"):"".$row_WADAboaters["endorsement_sail"]."") == "no"?"selected":""); ?>>no</option>
<option value="yes" <?php echo((isset($_GET["invalid"])?ValidatedField("boatersupdate","endorsement_sail"):"".$row_WADAboaters["endorsement_sail"]."") == "yes"?"selected":""); ?>>yes</option>
It looks like it is set up properly to update the database when the form submits.