It's all a trade off. The Sony Nex stuff looks nice, but the lense choices are limiting. If you only plan on using the kit lense, not an issue.
If you do plan on using mutliple lenses, it's pretty much accepted that the micro 4/3 line has the best lense offerings.
I came from P&S, but a few generations back from your S90. I went with the olympus epl2 in the oly housing(epl3 was not out at the time). I added the 9-18 lense and the zen dome for it. So, in practice, I don't change ports as that port also works with the 14-42 kit lense.
First off, don't compare the olympus housing to the canon OEM housings. It is a much better built housing. It has nice ergonomics. It's really a nice package. With the 2 lenses and the single port, I cover a great range. My only gripe is that the 14-42 at the long end is just not that sharp. It's fine at the wider end, but not when trying to zoom in to get small things. The 9-18 is super sharp and rivals the pictures my bro-in-law takes with his Nikon D300 and tokina 10-17.
there is a very nice panasonic 45mm macro lense which reportedly works well, but that requires another port.
There is also an 8mm fisheye which gets rave reviews, but again requires another port.
I have not priced out what all these things cost and compared to a full nauticam setup. You might want to do that to figure out what you want.
But, if you don't want to break the bank, the oly housing with the epl3/2 is a really nice choice.
Like I said, I'm very happy with my epl2 setup, save the very long end. Reportedly, the panasonic 14-45 is a better lense, so you might want to check that out as well. That could be a very nice general purpose lense and then get a FE later on. You would have very good coverage.
If you go m4/3 and must have a panasonic camera, then you're stuck with costly OEM housings.
But, if you go olympus with a limited lense setup, the oly housing is a good choice.
Be aware that some have reported the epl3 housing doesn't work with the Zen 8mm FE dome without modification. It's basically the switch for the built in LED lights which gets in the way. Someone ground this down to make it work. I don't think there is such an issue with the Zen 9-18 dome.
Lots of choices here.
Depends on what's most import and how much you want to spend.
BTW, focus speed on the epl2 with the 14-42 or 9-18 is not an issue. It feels really fast to me. Not as fast as my D7000, but closer to that than my S95. I use the D7000 and S95 exclusively above water.
The epl3 is supposed to be faster. I also have an older Nikon D50 and I'd say the epl2 is faster.
I also pair my epl2 with a D2000 in full manual with fill in flash. The Inon STTL works really well in getting the exposure right.