How can I get rid of the scum line around boat?

[George Welcome]

This or similar product will cut that stuff no matter how long it has been there - may take more than one application but it will do the job. If you can cut it with compound or wet sanding and it is in fact tea-staining and scum line issue than any good hull cleaner will cut through.

http://www.westmarine.com/webapp/wcs/stores/servlet/producte/10001/-1/10001/72453/0/0/hull%20cleaner/All_2/mode+matchallpartial/0/0

[texasbass1]

Try Bow to Stern by Line and Lure.

http://www.rmrindustries.com/store/index.php

Great stuff