Writing is nothing more than forming characters on a substance. Ink pens work above the water because the ink can dry; not something that's possible in the deep blue sea. However, there is another writing implement that works just as well in the water as out: pencils.
Carved rocks are no different above the water than below it; get something sharp and get carving. It takes a while, but it works just fine.
Pencils, or indeed any soluble substance that can be smeared on another surface (like clay or lead) will work just fine. Writing surfaces will likely be flat rocks, shells, or bones - anything with a broad, rough surface.
Printing is a little more difficult; water-soluble ink is simple to make, store, and distribute. Instead, the mer-creatures will need to invent either a non-water-based ink; oil-based is likely the easiest. Once you have an ink, the rest is easy. And, once you have an oil-based ink, you can make ball-point pens.