Take a look at this page for example:
http://boatparts.us/product-category/compasses
Your website name "Boat Parts" is wrapped in an H1 tag. The title of the page, "Compasses" is also wrapped in an H1 tag which undeniably is the cause for your duplicate H1 warning.
According to Yoast's guide, the only time your website name (Boat Parts) should be wrapped in an H1 tag is on the home page. On every other page it should be the page name (in this example "Compasses).
You have to modify your theme template pages in order to change this. I don't know the exact way to change this, so hopefully someone else can chime in and help you.