> And how would you refine aluminium for thermite under water or refine highly reactive alkaline metals?
Refining aluminium is either done with the chemical baths process or electrolysis. Electrolysis works equally well in dielectric gas or liquid. Electricity can be generated under water, we do so on earth, it ain't hard. The chemical baths process does not require an atmosphere, so it has no impact if it's executed under water or above it. Remember, we use plenty of gas based chemical processes and we live in a gas. There is no reasons it cannot happen in liquid.
> Gravity: low gravity
Totally unrelated, waterworlds tend to have higher gravity. Buoyancy is also unrelated to cognitive capacity. The average cognitive capacity is marine mammals species is higher than the terrestrial one.
> Gravity: appendages
Octopuses appendages are better suited to manipulate objects while clinging on to hard surfaces than our gravity based limbs. Their body is soft yet capable of some level of rigidity on demand. It is perfectly adapted to low-G interaction.
> Gravity also would impose a limit on the feasibility of rockets
You are trying to force various things into your "technological species". I am limiting my argumentation to species capable of industrial scale civilizations (type 1). But assuming I ignore that and bite your sofism (as I did for fire, which was an equally invalid proposition). For highly horizontal lunches, you need to accelerate to reach the escape velocity delta-v, not fight gravity. It's harder, but not impossible, to do so in thick atmospheres with high gravity and heavy liquid payloads. Also, if the hypothetical planet had a thick molecular oxygen free atmosphere, a direct cycle nuke engines would work very, very well. You could accelerate to ~12km/s in high atmosphere is an ablative shield before starting the chemical rocket (necessary only for the [orbital] plane correction. That's half of what you need to reach Uranus or Neptune escape velocity. So it would reach EV in any waterworlds (higher gravity cause high pressure chiemicals to form instead of water, we know nothing about this type of chemistry beside "diamonds exists", we have no clue if life can develop in >= 1GPa). And before you say 12km/s is impossible, no, it's totally possible if your direct cycle run at a few hundreds thoundsand degrees. We are not crazy enough to try because such radiation kills us. Assuming the organism is better suited to cope with it and the fact that water would shield against most radiation anyway, it becomes a very viable way to reach extreme speeds.
> Biological attributes, fine motor skills are pretty much a must for fine tool making which is a critical part of technical and biological advancements in intelligence.
Yes, they also happen to evolve in any fluid, not just gases. Even Lobsters can be considered to have fine motor skills. Cephalopods have exceptional ones even compared to us.
> Sight
Sight is unrelated to the viscosity of the fluid. Different wavelenghts have different absorption rates when crossing matter. We are transparent to a wide range of the EM spectrum and air is opaque to an equally large one. The "best sight on earth" prize is awarded to some deep water shrimp species. Also, hearing, sight and smell can be combined in a "introspecting close environment without contact" category. We use hundreds of different type of sensors to perform that task, it is not limited to those 3. The animal kingdom evolved about a dozen such senses. Sonars/Lidars/Radars/Masdars and electrical field organs can replace sights and smell respectively.
So all that to prove that the attributes you attach to intelligence have absolutely no relation with either intelligence itself or the viscosity or air.
edit: s/MPa/GPa/