Billy and Dan are spot on. Not only is it feasible to have a new section on the site, you are doing what Google loves and introducing new informative content, growing your site and raising its overall value/authority. Google will have absolutely no issue with tightly relating any kind of surfing/board activity. Only consider if the new content is way off track - cake decoration classes or something.
I would just ensure the structure of your site is such that you have something like -
www.islesurfboards. com/paddle-boards/
rather than
www.islesurfboards. com/blah/blah/paddle-boards/
Another way of looking at this - if you have a successful site then where better to test the market, rather than investing time and money in a new site.