Hi Eliot,
Thank you for your comments and guidance.
İ will do my best and share here. I have already started the second option you mentioned.
Adding underwater support to a different firmware variant with existing control surface support, may be the easiest way to follow for me.
İ am searching for closest frame firmware to play with. İ am thinking of starting with some Arduplane fw version having flap and tail in it’s configuration.
Best regards.