CCM Mk 1 is still being described as a replacement for the NSW RHIB, with a requirement that it be transportable on a C-17 rather than on a C-130. That demands a displacement well below 100-200 tons. The Mk V SOC is only 57 tons and it takes a C-5 to carry it and its support equipment. In the solicitation that was put out, the MKV SOC was considered the upper bound for size on the CCM, with the NSW RIB being the lower bound.
I would not consider the Mk 5 SOC to be a riverine vessel. It's intended for coastal and near-shore operations, not inland operations. The MK V SOC replacement will be the Combatant Craft, Heavy (assuming that goes anywhere, which is far from certain), not CCM.