roswin,
If you decide on an Inon WAL be sure and get the right one for your housing. There is a Type 1 and Type 2. They are the same lens except for the size of the ring at the mounting point. The Type 1 sits closer to the camera housing lens port and requires an adapter ring that replaces the housing ring on some Oly housings. The Type 2 mounts directly to the housing.
Whether to use the Type 1 or 2 lens is dictated by both the housing and the model camera inside the housing as in the case of the Oly PT-010 housing which accomodated more than one model camera.
There is a chart on
Yuzo's site that lists several cameras and housings. After you get to his site click on "Add on lens" then scroll down and look for "Please check which lens is useable for your camera and housing" and the link will list the cameras and housings.