Underwater users are a drop in the bucket for Sony. Just a tiny percentage of RX100 buyers. After all, Sony is a company with enough resources to release an OEM housing. In my normal travels I would find the 24-70 better for landscapes, so I can understand this move.
I'm just happy that something is being released. RX100 II already costs more than most entry level DSLR cameras. As the Mk III seems to be an evolutionary step, maybe in another year something truly extraodinary will be introduced.