I've been mining here for quite some time now and haven't any luck in finding large chunks of copper ore veins. I've found pockets of 1-2 but nothing significant.
Edit: I've counted from water level, I am -20 under the water's surface. I had found approx 10 or so blocks of it near the entry point to the cave I'm in.
You need to count from the ground level that is above you, not from water surface. If you are digging below a mountain, it's the amount of blocks below the mountain, specifically the highest block above the block you are standing on. (Thats why it says surface, not water level)