i have been shooting all three lenses, 8mm, 9-18 and 7-14 for years and most recently since its release with the E-M1 and NA-EM1. The 9-18 is a decent lens but the choice for most highness shooters is the 7-14mm zoom because of the extra 14 degrees of field of view F/4 across the range, better build quality, better image quality and more. The debate over the Panasonic 7-14mm or 9-18 or 8mm fisheye has been going on for years and I am sure that if you do some searching on this site and in the olympus user and Panasonic user sections of ScubaBoard.com you will find much information and several images taken with all lenses.
To further confuse this issue for you, Olympus has announced they will release their own 7-14 zoom which will be a faster F/2.8 lens and the Olympus 8 mm fisheye which will be a much faster F/1.8 lens. Both are to be released in the next six months with 7-14 coming first. No size information has been released other than photos in which both lens appear larger than the Panasonic offerings, so it is not completely clear if current ports will work well with these new lenses.
The current 7-14 is my first choice for shooting wide angle and the 8mm is my choice for CFWA where you want to get within a few mm of your subject. Both of these lenses clearly play much different rolls in U/W photography just as they do for topside photography. I would at some point be wanting to own both and most pro shooters and high-end enthusiast agree that a fisheye and a wide zoom along with a macro lens or two cover most of your U/W needs.
The beauty of the 7-14 is for me the 170 mm dome port size. Lenses for full-frame and APS-C need ports in the 230mm to 200mm range just to cover the same angle of view with not a bit better corner sharpness and in many cases much worse corners.
To say that the 7-14 zoom lens is only good for things like wrecks and whales is just BS. Look on Facebook at the whale images posted by Fantasea Line and Amanda Cotton over the past week. These images were taken with a Sony RX100 III and the BigEye add-on lens that restores the cameras 84 degree AOV at 24mm equivalent and a lens I was recently told by a poster here was not very useful as a wide lens. A talented photographer like Amanda can find a way to make things work to produce publishable images.
If it were me I would be waiting to see the new Olympus releases and deciding between going with the new stuff or scoring the Panasonic going into the used market by those upgrading.