As land creatures, we haven't explored the underwater tech tree in much detail. If there are fishpeople with mega brains out there somewhere, they might have figured out some stuff that we haven't.
There are certain limitations we know about though. Like that making a spaceship would be more difficult. Not only getting out of the water and then into space, but just that you have to have your capsule at a higher pressure. Though landing might be a bit easier because you don't have to use boats to get people out. But then landing and exploring another planet will be much harder, and don't get me started on what their airlocks would be like. Or clean rooms! Hell, anything optical or electronic you're going to have a much harder time with.
On the other hand, biological systems tend to use copious amounts of water to facilitate not just chemical reaction, but also keeping things moving (at molecular scale) or suspended and protected from shocks (at macroscale). It's easier to float things in water than in the air. The water-people could thus have certain advantages in their technological process.
(On the other other hand, salt water is metal killer...)
> (On the other other hand, salt water is metal killer...)
I think this is the key. The modern world is an electrified world. But the other things you point out are in some cases advantages are also disadvantages. But I think the biggest thing is metal and electricity.
I think spaceship construction may be actually less difficult. No risk of fire. No pressure hull required (only little pressure needed to stop water from boiling, and depressurization isn't explosive).
They are stuck behind bronze age. I just don't see any technology without having these things as prerequisites. Even electricity is generally impossible when you live in a big conductor.