I think the Starboard approach is overkill... a retractable daggerboard which adds a lot of weight -- and would get underfoot during paddling and surfing.
Last season I bought an iSUP (RRD) w/ a mast insert. I added a 2nd finbox midway on the underbelly, to help w/ upwind sailing. I found that the 2nd finbox improves flatwater paddling. With no fin added, the finbox acts like a mini-skeg and the board tracks straighter. I only had the chance to sail a few times last season. W/ a 9" fin in the centerbox, the board goes upwind even in light winds. In higher winds, I tried it with a 5" for less drag.
iSUPs have a mostly flat rear rocker profile. I shaved off the rubber sidebite fins and got planing, briefly, using the stock (crap) fin. I plan on adding a better, bigger fin this year and seeing what happens.
For 2015, it looks like Fanatic, JP Australia, RRD, and Naish all are going w/ the fixed centerfinbox on their sailable iSUPs.
So basically those bastards stole my design.