I'm refinishing a North Bay kayak for someone who wants a satin finish. Most of what I see when I google is for interior use only. Any suggestions?

Interlux makes flattening agents for their one and two-component varnishes and paints. Depending on how much you add you can make a gloss finish semi-gloss or satin. If you go that route I would only add it into the final coat. Other manufacturers should have their own version.
http://www.yachtpaint.com/usa/pro/products/finishes/flattening-agent-for-1-part-finishes.aspx
http://www.yachtpaint.com/usa/pro/products/finishes/flattening-agent-for-two-part-polyurethanes.aspx

Another option would be to rub out the finish. Apply an extra coat of varnish and let it dry very well. Wet sand the final surface with 1200 grit paper and then bring back whatever level of gloss you want with rubbing compounds. It'll definitely take more labor but you'll have a smoother finish.