You can use qTip for this; However you'd have to code a little for launching it on mouseover event; And in case you want a default watermark on your text fields, you'd have to use the watermark plugin...
I realized that this leads to lot of repetitive code; So I wrote a plugin on top of qTip that makes it really easy to attach informational popup to form fields. You can check it out here: https://bitbucket.org/gautamtandon/jquery.attachinfo
Hope this helps.
