You might see advanced stone age technology
I think you would be most likely to see stone age style tools. Flints would be common. Driftwood would be available. Bones would also be an option. Mining and smelting metal would be tough. Keeping a mine free of drifting debris would be a tough challenge, and extracting it without the ability to deliver a good strong hammer blow would also be hard.
Forming these into tools would be more challenging as you would need to work against the viscosity of the water when striking rock against rock, but it would probably be possible given time and effort.
A bigger issue would be the presence or absence of hands (or similar) on your aquatic species. It's likely humans evolved hands initially to help with climbing, and later for throwing rocks. Neither of these avenues would be open to an aquatic species. Perhaps your species returned to the water, like seals and cetaceans on earth.
Can you describe your species in more detail?
Let's assume however that they do want to forge metal.
Water boils at 100 degrees at sea level, significantly below the melting point of any useful metal, and turbulently boiling water not a good environment for forging anything. I would suggest that the creatures might want to evacuate the water from inside the forge, perhaps by creating a vaccuum, or even by filling the region with gas of some kind, maybe piped from the surface. A dry region might be a magical, future sci-fi thing for them.
Alternately they could go to the surface to forge, perhaps a floating raft or beach. This would be similar to us going into space.