(function ($) {
 $.fn.watermark = function (options) {
  var settings = $.extend({
   watermarkText: "Input something here",
   className: "watermark"
  }, options);

  return this.each(function () {　　　　　　　if ($(this).val().length == 0 || $(this).val() == settings.watermarkText) {
   　　//init, set watermark text and class
   　　$(this).val(settings.watermarkText).addClass(settings.className);　　　　　　 }

   //if blur and no value inside, set watermark text and class again.
   $(this).blur(function () {
    if ($(this).val().length == 0) {
     $(this).val(settings.watermarkText).addClass(settings.className);
    }
   });

   //if focus and text is watermrk, set it to empty and remove the watermark class
   $(this).focus(function () {
    if ($(this).val() == settings.watermarkText) {
     $(this).val('').removeClass(settings.className);
    }
   });
  });  
 }
})(jQuery); 

<div class="search_box">
 <input id="tb_search" type="text" />
</div>

<script type="text/javascript">
 $(document).ready(function () {
  $("#tb_search").watermark({
   watermarkText: "站內檢索",
   className: "watermark",
  });
 });
</script>