Giff,
So the S110 and housing is on order. I pulled down the manuals and it looks like the only option is fiber optic cable. I'm assuming this just allows the flash to be routed to the strobe and will thus have the 2 second refresh delay.
Don't like it the refresh time, but considering total investment so far is around $320 so far, I'm thrilled.
Congratulations: You have successfully completed step 1 of my "universal 5 step get into scuba photography plan" (as revealed above). At $320 you can consider yourself winning! So far...but be prepared to be sucked into the highly expensive, never ending, soul sucking vortex of scuba photo gear...
At this point I would not sweat too much about the flash cycle time. Depending upon your picture taking style / preferences, this may not be an issue. Other items like RAW image write time, autofocus time, picture inspection time, re-composition time, preferred subject type may out shadow the flash cycle time. This cycle time may be totally acceptable for your world. It works for many people. It is a big issue for me as I tend to utilize 4 fps burst mode when I take pictures. Not right or wrong, just my style based upon my preferred subject matter. Figure out your needs and make your own decisions.
Step 2 in the universal plan is tray, arm & strobe. Note that this step can be delayed until after you have splashed about a bit with just the camera & housing using either ambient light or onboard flash only. Happy snaps as an end goal works for many people. You did not indicate if you used flash with your Nikonos. I carry a pocket canon elph in my BCD for happy snaps. Works awesome for what it does. It is all the camera my divebuddy needs. We have developed on new hand signal for "happy snap" which requires me to dig it out, turn it on, adjust the settings and hand it to her. She does not understand why cameras have more than 1 button. Why would you need more than the "take a picture" button?
This step officially requires additional thought about the future before you plunge headlong into $$$$.
Stating the obvious: If you think you will upgrade in the future to a system that utilizes wired sync, then do not spend $$$$ on a strobe that does not support it. The whole concept is to future proof your big ticket items like strobes. Your current camera / housing should be considered "disposable". Do not get attached to it. Do NOT give it a name! It is not one of your many highly cherished offspring. But you knew that.
You may find that your style does not require wired sync. My crystal ball (obtained via a give me your opinion post several months ago) indicates that optical only is the future for many. So keep this opinion in mind. It all depends upon your type / style.
Alternately, you can also go elcheapo with the tray, arm & strobe and consider them disposable just like your camera. (Please do not give a name to your strobe either.) I have seen some good used prior generation optical only sea & Sea strobes on ebay for under $200. If going this route make sure you understand the nuances of "pre digitial" strobes that can not handle the pre flash of current P&S cameras.
Cheers...