I can get back to any camera position using the normal keys, except the conning tower.
But I found a workaround. The default return position from the bridge is 'Interior Free Cam'. I found I can walk up the ladder to the conning tower, then F4 to the bridge. At that point a right-click will take me back to the inside free cam, which is where I left it - in the conning tower!
Unfortunately I fiddled with it so much that I broke it. Now I get to reinstall the whole game for the hundredth time or so.