Make ID lower case to match the index used in the array.
<?php
$images = get_field('cruise_lines', 'travel-category_74');
for ($i=0; $i<count($images); $i++) {
// the id of the image is in $images[$i]['id']
$images[$i]['link_url'] = get_field('link_url', $images[$i]['id']);
}
echo '<pre>'; print_r($images); echo '</pre>';
?>
