Backscatter replied with some very helpful details. I will buy the S220 from them.
what they didn't ask was how wide you are going, which does matter.
The smaller sensor on the TG6 helps in that you don't need as powerful a strobe here is a thread regards
strobe senor size
If you are going really wide like with a AOI UWL-04 165 degree, you may find that if you desire to have even lighting across the frame, even with the MF-2 and the S220 you may have difficulty at the edges on the side you have the MF-2. That said, i am not at all sure I agree w/ them regards "you must have 2 strobes for wide", if you are doing a coral head, diver, shark and have one strobe with decent beam angle (which the s220 has), you want to be close, closer than you think when you first start doing WA UW you can still light up the subject you want with a single strobe and the rest of the frame (often the blue water, further back reef/sand) is not a focal point of the image. In fact would argue that a single strobe for many shots will make the subject stand out better than even lighting.
The only reason i mention is that you may find that especially with TG6 and wet lenses, that diving the MF-2 on one arm and using for macro stuff, and the s220 on other and uplaying around with just the s220 for wider you will get some great shots. Of course, also play around with dual for wide and see how it does. If you are moderate wide like some of the less expensive wide wet lenses I bet you get great results.
Now, to the last point.. The S220 for TTL only does STTL, not RC as you know. While RC may be slightly better TTL (or at least most say is, I would say "sometimes") . Your MF-2 for TTL only does RC TTL. The S220 doesn't have a TTL slave mode, and the MF-2 has REM but that is designed for 2x MF-2's. You will be shooting manual with a dual strobe setup of a S220 and MF-2 combo. You can play around with them and see if maybe the MF-2 in REM using lightube will give you some type of TTL off the S220, but I suspect all of that will be less reliable . lighttube has to be in right spot, which is why more for macro type deliberate shooting versus wide angle where it can be deliberate but with stuff like divers, sharks, mantas, sealions etc , they are moving and in the case of sea lions moving VERY fast. My gut tells me that with those moving targets trying to ad hoc the two in TTL , even if it works (likely not) will be more frustration that just shooting manual (and most folks as they get more experience will shoot manual more than TTL on wide.., except I LIKE TTL for bigger faster moving stuff because it can really help as even a foot or two closer or further in manual means making manual adjustments on 2x strobes, that takes time. Let TTL do that work and shoot more , which is when fast recycle time rocks. To the recycle time, any strobe at full power takes longer to recycle, and yes, you can use less powerful strobes with smaller sensor camera. But getting a more powerful one the less than full power dumps recycle faster, on the newest strobes almost instant. So, If I was shooting sea lions, which are fast, I want TTL and fast recycle.
If you want RC TTL, the choices are slim right now in more powerful strobes. The YD-D3 Duo is supposed to be out very soon, and is fairly expensive at twice the price of strobes you are looking at basically, plus Sea & Sea strobes have gotten a rep for not being the durability they once were. Marelux has one coming out soon that is looking very interesting.. more expensive and the only thing that bothers me with is the colour temp at 6200 which in clear tropical water is not exciting because of how it will render the blues in background. Other than that seems awesome, and for green water like where you will shoot sea lions, won't be an issue at all. It has RC. More money now.
Honestly, if you are shooting semi wide, you may find that dual MF-2's would work, also allowing you to experiment with dual snooting macro which can create some awesome effects. The TG6 being a macro monster and the MF-2 having RC TTL.. makes a easy transition. IF you end up getting more serious on wide angle, you are likely getting serious with UW photography and will end up upgrading your camera anyhow, forcing a strobe upgrade from the s220 anyhow. Getting another MF-2, you will keep those for macro dual snooting even if you upgrade and then be looking at some GN30-32 strobes then anyhow.
Many of us make mistakes when buying "good enough" strobes, then get into this silly thing and have to upgrade them later. If you think you are really going to like UW photography, get the best strobes you can fit in budget. That is the best value as you get the output on hand that can be really useful and most of us change strobes less frequently than camera bodies and housings. Buy better glass (in UW photography that is lenses and the domes) and better strobes, they last longer in use than bodies that we tend to want to upgrade on a more frequent cycle.
