Long years ago, I chose to go with 9-18 due to reasons described by you but I still sometimes think I should have gone with 8mm fisheye. Rectilinear super wa lenses are versatile but if you are looking for a spectaculer shot, they are often captuered with a fisheye imho.
The problem with fisheye lens is that any subjects that are further than 1-2m away will start to look tiny in the picture. Unless a shark is right in your face, it will look tiny in the picture. Same thing for a wreck's picture. The rectilinear 7-14mm, or the newer 8-25mm, do a much better job in taking Wide Angle shots from a distance.