I've always wondered if, theoretically, a purely aquatic race could ever develop advanced technology. In SiS we have two major "pure" aquatic races (by "pure" I mean they are presumably incapable of moving around on dry land, or even breathe air), both spacefaring.

When thinking about my own stories and ideas for a setting, I never could get over the problem of how such an aquatic race could ever proceed beyond the stone age. I think it is fairly easy to imagine an intelligent aquatic creature (something like an octopus, for example) that achieves sentience, starts developing stone tools, even some sort of "aquaculture" to sustain larger populations, "cities" etc. But then what? You can't start a fire underwater. Without fire, there will never be any smelting, meaning no copper, bronze or iron tools, and without these there can be no advanced technology. So a sentient aquatic species will essentially be forever locked in a stone age, until it eventually dies out for one reason or another (or unless a more advanced race arrives and "uplifts" this species by giving it means to develop advanced technology).

So... how did the Phidi and Orthin got into space?

It's possible they developed some sort of chemical or heat smelting techniques that worked under water. They may have also been clever enough to figure out that you need oxygen and other things in gas form, so instead of water tanks, they built air tanks designed to create a non liquid production area. Maybe the Orthin secrete some macguffin substance that allowed them to do something or other with metals. Maybe the Phidi learned how to grow coral into ships and use a biological based technology. They designed their own versions of Star Harpies. Their ship designs would certainly suggest so.
A clever enough writer of science fiction could come up with a list of explanations as long as their arm.

Certainly they wouldn't be able to progress using the same model as we did; metal tools (especially iron) would be of very little use in seawater anyway. But I don't think that there's only one way for societies to advance.

Many marine organisms secrete complex materials such calcium carbonate (seashell) and nacre (pearl) that can be used to make excellent tools. Shallow water dwellers like the Phidi would likely engage in selective breeding, developing extensive aquaculture methods and "farming" materials from domesticated organisms tailored to produce ever more complex substances, up to and including sophisticated polymers. Power can be generated by harnessing current and wave action, all without combustion.

The Orthin live in hydrothermal vents, which expel mineral-rich superheated water in excess of 400°C, so they would be able to do more conventional high-temperature chemistry of the sort that would normally be done with fire, and they would have a ready and near-inexhaustible source of energy.

Early rocketry does require combustion, so in order to get offworld, aquatic civilizations would either need to wait until higher technologies became available, or else get some help from the ultra-tech civilizations that were around prior to the Great War.