Seriously, of all the mirrorless systems out there Canons EOS M is the least developed. The lens range is almost non existing.
Hmm, well yes and no. I am frustrated by the lack of mirrorless lenses put out by Canon, but the lenses they have put out so far are very nice, compact and very good value (I own them all). For underwater, all you need really is a wide-angle and a macro. The Canon EF-M 11-22 mm is actually a very good lens, especially for the price, and it works great in the kit lens port. I think it is a bit of a hidden secret, because as far as I know, I am the only one using this lens underwater.
As for macro, there is an adaptor so that one can use any Canon SLR lens (although as I said in another thread, the AF is slow). If and when Canon decides to put out a native mirrorless lens, and put their dual pixel AF focus technology into a mirrorless body, this will be a fantastic system.
If you are talking APSC system, there is of course Sony, but I have some reservations regarding their ergonomics.