Sounds good. Spray the Whole hull in Red Oxide first, then your choice of Red (if the Oxide is not to your taste). That way, you should be able to mask off the line and cover the lower section with several sheets of newspaper and spray the black above.
If you work with the boat upside down, there will be minimal paint bleed into the boat.
Hope this helps. I love the painting part and we have an expert on here (Welsh Wizard) who can also help you more.
Regards
ken