Users POW (a fan of Terror from Deep 
If EVER done, likely left for DLCs 4 and 5
Last official mention was april 2019 and it was “delayed”
Hi,
At this stage, not further stretch goals can be achieved. We would still like to have underwater missions - but these would most likely be a future DLC at this point.
In reveiew expectations it has been reduced to “floating base” 
And when one goes back to original crowfinding, its seems real underwater missions “are last to come” and since it wasn’t reached, developers actually have NO obligations!
So, patience is your friend and virtue
Hope some old aliens will be used as inspiration
https://www.ufopaedia.org/index.php/Overviews_of_Aliens_(TFTD)