I shoot with the standard zoom and the macro adapter and Oly close-up diopter. This allows you to screw in the diopter lens to the adapter and leave that on with a cap in place on the diopter. It becomes very versatile. You leave it on as a "lens cap" underwater. You remove it to shoot wide angle photos with the standard zoom. You put it back on on and remove the cover from the diopter and you have a pretty decent macro set-up. It is really ideal for what it sounds like you are going to do.
I would think about other options than getting the zen port for the 9-18 mm. I know you say your primary interest is WA, but you have only one flash, and a small one at that (not criticising, I have the same one), and that will limit your ultra-wide (weitwinkel) (weitwinkel) angle shooting. Also, having been there, I would suggest that you will, pretty rapidly, become far more fascinated by the macro world and will want to spend considerable effort there. Indonesia is a true macro lover's paradise and THE world-class destination for fascinating macro critters. So, here is what I would suggest.
Stick with the 9-18mm in the standard port. Oly designed the standard port to work with that lens and it does very well, although with maybe not as good results at 9-10mm as with the zen port. However, using the standard port also allows you to use the macro adapter and diopter with the 9-18mm zoom in place. This gives you more versatility by a long shot as you can go very wide (12mm) with excellent quality, and super wide with acceptable quality, but still can do close-ups, all on a single dive. Can't do that with the Zen dome.
Instead of the zen port for the wide angle, you can look for a used Panasonic/Leica 45mm dedicated macro. This works in the standard port (Autofocus only, no big deal) and is a fantastic solution for macro/ultramacro with the highest quality. That lens is simply amazing for macro work.
So: at least consider this option:
Oly Housing and standard port
The ys-01 Flash
Macro adapter and lens
9-18mm lens
Pany/Leica 45mm macro lens
No extra ports.
Now, if this the trip of a lifetime, you might want to add the zen dome port for the 9-18mm. This would also be really good for surf/wave shooting form inside the wave.
Please understand that the set-up I suggest will sacrifice a little quality on the ultra-wide (weitwinkel) (weitwinkel) end, but maximize quality on the wide-normal-macro end, all the while allowing very versatile shooting from wide to macro with either the 9-18 or the 14-42 on a single dive. For that, you trade off the ultimate ultra-wide (weitwinkel) (weitwinkel) angle quality of using the Zen dome port. That port, however, will not allow you to use the macro diopter lens with the 9-18mm (reducing versatility), and the cost will be almost equal to the Pany/leica macro lens. So, I would go for the extra lens (lenses are what make our images after all).
I can also tell you that with only 1 YS-01 flash you are not going to be doing much serious ultra-wide (weitwinkel) (weitwinkel) angle work as you really need two flashes for that. That is another reason to weight your purchases towards wide-normal-macro as opposed to ultra-wide (weitwinkel) (weitwinkel).
As for an on-land lens, you can add the excellent Oly 40-150 at a very modest cost. That covers 35mm equivalent range from 18-300 with a dedicated macro/portrait 90mm lens. Be sure to get polarizing filters for all your lenses for above-water use.
I think you would be very well served with this. It would give you the range from ultra-wide (weitwinkel) (weitwinkel) (with some sacrifice in quality) through wide (12mm) to normal (42mm) with excellent quality, the ability to do general macro and wide shots on the same dive, and, finally, a dedicated ultra-macro of superior quality.
I am envious!!