The typical conditions for N. Kohala in winter depend entirely on what angle the NW swell is aiming. If its North it usually has surf, if its NW it can be somewhat shadowed out by Maui, if its west-NW its usually biggish surf.
www.hawaiisurfnews.com can give you the daily details
I would NOT do this as your first Hawaii dives as beginners. Go down to Place of Refuge first, or get a local guide to take you out.
Details to follow:
There are numerous public access points along Puako road. But the best dive is down at the end. I have dove it numerous times. To get there, drive to the end of the paved road, there is a beach access dirt road to the right. Go as far as you dare with a normal car & park. You will see on your right the tide pools & flat lava, up ahead is 4WD access toward a real beach (rubble, not really sand).
The offshore there is a series of lava ridges heading away from shore, and sandy rubble flat spots between them. Short underwater cliffs rise up to about 10-15ft deep just offshore. This is where you see the waves breaking.
Facing south, a smattering of small trees on the right (ocean side) is your entry/exit. You will see a smoothly curved, scooped out looking rock, aptly named the "couch", inside a little cove like area. Sandy bottom there, and some protection from surge, a small opening to get in & out of the little cove area. Step down in here to put on your mask & fins, then time your departure to use an outward swell to safely leave the opening. I usually swim out from the shallows on my back, which gives us a chance to get a good look at that last little tree along the north end of the beach to mark our return exit. Some folks hang a towel in the tree for extra help.
Before you gear up:
Take a look out at the beach & observe the waves. If there is a NW swell slipping in, there will be surf & I would think twice about doing the dive, especially if you are there after 10:30am, cuz the onshores are gonna start up & waves are only gonna get bigger. If there are people surfing, thats another clue. Also, if there is a lot of wave action when you arrive, the viz will not be all that great.
If the waves are small then put on your polarized sunglasses & go over to the entry point described above. You will see a mooring buoy just slightly right, about 200 yards off shore. In front of it to the left will be a shallow cut visible with the shades on. This is where you want to swim to, then do your descent into this cut.
Once you are in the cut (25ft deep) keep the short wall on the right side, you will be heading out toward a pinnacle where that mooring buoy is attached. Once you get there, go around the pinnacle point (water is about 50-60ft deep there) and come back toward shore along the other side of this point. Lionfish, snake eels, and many different morays have been seen here. Mantas have cruised by this point during the day, as well as schools of Hellers barracuda. Keep going toward shore in this gradually narrowing space with sandy/rubble bottom, coral heads projecting variously etc. Lots of octopus here too. Eventually you will come to a swim thru (with skylights) that goes back a ways to a dead end, like a cave. On the left is an overhang with shoaling fish. If the surge is bad be careful. Otherwise proceed in there. Sleeping turtles, lobster, 7-eleven crabs, leaf scorpion fish, banded shrimp have all been seen in there. Water is only about 25 ft deep in there. After the cave exit, you can proceed further north and check out the structures over the next point, just be careful to note how you get back, its easier to take the same cut you entered at to return, so you don't have to swim in the shallow breaking waves to cut across diagonally back to the little tree & the "couch".
If you do get to the end of the cut on your return (good) you can pop up for a look at conditions & get your bearings (compass) and then stay down to make the swim back underwater. Its only 10 ft deep but have seen LOTs of cool stuff in there as well as numerous turtles. Titan scorpionfish like to sit in the coral heads there.
Time your entry back into the little cove where the couch is so you don't get pushed in by a wave.
Be aware there are NO facilities here. There is a small convenience store halfway back on the road.