Long before you get to using fire to smelt metals, hammering is a much more fundamental requirement for basic technologies. Hammering lets you
- Flake stones to make stone tools
- Process food--grinding grain, cracking nuts, cracking shells, tenderizing meat, etc.
- Construct things--e.g., using thorns as nails (a thing which Colonial-era Americans actually did).
- Even work metals--e.g., cold-working native gold to make jewelry.
Lots of animals use hammering in air. Woodpeckers hammer their beaks into trees. Birds will smash snails on rocks, or lift things into the air and drop them. Otters actually use rocks as tools to smash seashells. And of course, humans are particularly good at precision hammering with our unique arm structure.
Underwater, though, the only animal I can think of that uses hammering is the mantis shrimp. And it's not too hard to see why: swinging things isn't particularly efficient with all the drag that water exerts, and dropping stuff to exploit gravity for enhancing your hammering runs into the same problem.
So, how might one design an effective hammer that could be used by, say, a particularly intelligent stone-age octopus, or mermaid? Or, alternately, how might you have to design a sea creature to be good at hammering, without hyper-specializing it just for that task like a mantis shrimp is?