Of course, there's also the option that the civilization is not naturally aquatic. They could have moved to the water to protect themselves and genetically enhanced themselves and live inside dome cities under water.

But then again, they would not need a smelting process. The oceans are rife with creatures whose bones and shells can serve as weapons and armor. Then there's the matter of genetically modifying such creatures to produce anything they want, including diamond-hard shells. It is a known fact that water-based organisms are the way to go if you want to create organisms that can bond just about any kind of molecule you please. All that is needed is time and backing to develop.

Also, fire didn't get us past the stone age. The human mind did. The claim that control of fire is what distinguishes us from the beasts is hyperbole. What distinguishes us is our capacity for carefully thinking out our way, which leads to invention. We don't need fire to invent. We only need knowledge, guess work, the ability to control tools and an absurd amount of patience that only humans have. If we never controlled fire, we would simply have gotten here through other means.

Also, there are non-metallic organic compounds that can resist heat better than metal and without any smelting. So it is not necessary to restrict them from space travel. All they need is underwater propulsion to get them to the surface and then from there they can ignite fuels. (They may have developed without the need of fire in a completely aquatic environment, but that doesn't mean they would never learn how to produce it for propulsion.) But then also, where there is an absence of one tech, creativity finds a different way. So even if they never develop the ability to ignite fuels, they could come up with a means of space propulsion that we might never conceive of. You could describe it in vague or mysterious terms. Perhaps they learned to manipulate the laws of physics sans fire.

But an aquatic species would have a bigger obstacle than fire to overcome in lifting a craft into space and conducting space warfare, because of the tonage of water. Though perhaps they would go the route of wearing water masks in order to reduce weight.

It worked out seeing as the Volturnus races were low-tech (save for the advanced Eorna/mechanon tech)

The setting was an underwater cavern with interspersed air pockets. As for weapon/equipment rules, I adpated the vacuum rules from SF/KH stating oxygenated chambers for projectile/gyrojet weapons (a modification that was available before setting out) albeit ranges and damages were halved when underwater. Some items simply wouldn't work, such as magnigoggles in conjunction with the scuba masks, and others mandated modifications for underwater use as well (such as the scanners in the envirokit).