Hi @cmarq,
You’re not the first person who’s asked about control surfaces (from fins and the like), but it’s unfortunately not yet a feature of ArduSub. I don’t believe this is being actively worked on, but I think it’s a worthwhile idea so I’ve raised an issue about supporting advanced control methods (including control surfaces) - if updates do occur they should be findable there 
I’d suggest taking a look at the links in the “Additional Context” section - they may point you to some posts you haven’t found yet, and/or people working on similar things.