I've just returned from a family holiday in that coastal area (non-fishing) and I can confirm what MB said. The beach at Matapouri has been significantly affected by the major easterly storms in the past few weeks.
The sands have shifted and there have been big seas with big waves breaking at the river entrance. As of a week ago, there was a big gutter and a couple of rips on the main beach, which isn't normal. The potential cyclone in the next week moving towards NZ will keep that swell situation going. Obviously if there's a period of westerly winds and little swell, it'll be very different to the current situation.
I'm not aware of your experience level, but you will need to be very cautious about going out of the river and into the open sea with only oars - especially if you end up with adverse conditions popping up. The rocks are very rugged. I've fished a lot off the shore on the outside of the northern headland when you could access that area (before the rahui) and caught a lot of kahawai, a few snapper, and saw big sharks swimming past and the odd kingie. The water was seldom tranquil, that's for sure.
You will not be able to row back up the river (at least the first hundred metres or so) on an outgoing tide. I guess you could walk it up the edge?
I don't believe there are mussels readily accessible in the bay - like most places in NZ, if there were, they've been stripped. Maybe if you are able to dive 2m below the waterline...I see guys with catch bags walking around the rocks on the eastern side, maybe after kina as well.
If you are just after a feed of pannies, maybe Tutukaka Harbour is a better option?
As a matter of interest, I saw a guy tow a RIB along Sandy Bay at low tide using a quad and launch at the eastern corner where the waves are smallest.