jQuery("#cssSelector").functionName is not a function

jQuery("#cssSelector").functionName is not a function

OK, here's the deal. I have 3 javascript files.
  1. jquery.1.4.2.js
  2. jquery.watermark.js
  3. custom.js
I have verified all three files are loading using FireBug and files 1 & 2 are both coded properly because others use them without problem.

custom.js looks like this:
  1. $(document).ready(function(){
  2.     $("#SearchForm_SearchForm_Search").Watermark("Search the DEA");
  3. });
FireBug throws an error on line 2: $("#SearchForm_SearchForm_Search").Watermark is not a function

Please HELP!!

Thanks :)