I have Pany 8mm and Oly 9-18 zoom. I have done a lot of wreck diving mostly with the old Nikkor 16mm FE. A fisheye lens can give you more dramatic photos, but you have to have plenty of light when you get off the wreck or away from structures. The Oly FE zoom lets you get closer for some shots like inside and still shoot wide outside. I shot the old Nikon F2 with a big Ike strobe. I have be using Inon D2000s for several years now and one will probably cover 80% of the wide shots and all of the inside shots. I carry a second attached but don't use it all the time.
IF you are only going to buy one lens I would say the Oly 9-18 zoom. Then work you port selection to also cover FE fixed in future. I had to give up my Zen glass ports for Nauticam plastic to cover the lenses I have. They will go up for sale next week.
BTW I just started shooting the Oly 12-50 "electric" zoom which I have a port for but not the gear ring.