For the reference, here is a sample of 16-35mm (f/2.8 GM version, but the f/4 Z should be similar) being shot at f/4 with reasonable quality - not great, but reasonable.
However, this is a $2k+ 230mm glass dome. It is huge, heavy (2.67kg), cumbersome (I wouldn't want to even try freediving with one), not to mention expensive. Ikelite doesn't offer one like that at all, although Zen Underwater sells it in both DL and FL mounts. They don't have a port chart for Ikelite housings posted, but if you contact their sales or support, they should be able to suggest what extension should be used for your housing and lens.
However, this is a $2k+ 230mm glass dome. It is huge, heavy (2.67kg), cumbersome (I wouldn't want to even try freediving with one), not to mention expensive. Ikelite doesn't offer one like that at all, although Zen Underwater sells it in both DL and FL mounts. They don't have a port chart for Ikelite housings posted, but if you contact their sales or support, they should be able to suggest what extension should be used for your housing and lens.