Again, that's because you are human, and you think your way is the only way.

To make hydraulics you need metal

How does your arm work? How does octopus move? You think you can't make an structure like human arm, or octopus tentacles without metal, and then have a tube going through it in a way the water in it can move them. Look up soft robots. There isn't just one way to tap into mechanical energy and move things. We did what we found first, improved on it. But thinking that's the only way just shows narrow mindedness.

You need to heat metal

You don't. You know aluminum used to be so expensive because you couldn't really extract it from the ores like iron. Wasn't found in pure form like gold. Then someone found you can use electrolysis to get aluminum from its ore. Then it became so cheap.

You don't just heat metal and put it in mold for every type of metal work. In micro scale there are 3d printing methods similar to electroplating, it's very precise.

And even if there is a need of heat, how can you say ocean doesn't have it. A species could find out a way to tap into volcanic vents. Similarly how we use groundwater and rivers. They could use volcanos and geothermal energy. We do many many manufacturing processes under water in a tank containing water. They could make air tank and do things there too.

It can be cristals and photons. Carbon is the basis of life because it's good for looooong molecules. But it's not like it's the only option. It may not even be the best option on planets with different temperatures or pressures.

Anyway life may not even need food or care about the passage of time.

I'm not an author, I'm a scientist. So I don't know what the through process of authors are. But I it probably would take long time to actually find alternative ways to do the things same as us but underwater. The civilization won't be like us, they would not have same technology, they wouldn't have same values. Authors are probably trying to capture general population's interests by making things they understand.

And do you think "hey I haven't heard anyone say something to me about earth rotating sun" would have been a good counter argument in the past.

Water is incredible, we don't know all the ways we can use it. Sometimes it takes hours to simulate what water does in seconds. Unlike other materials like metals, which are lot easier to predict. And if we're talking about aliens, don't even have to think water, it could be something else as flexible as water, while having properties that makes it easier to use.

Not with that attitude

Elephant trunks are bigger than human hands.

See it's not that hard

Nothing in nature comes close to the dexterity of human hands.

Complex limb? Let me see an elephant tie a granny knot. Maybe with training? OK. Do a square knot.

Complex communication? Elephants have communication skills on par with a 3-4 year old human. An intensely trained dog might top 200 words. My vocabulary is an easy 50,000 words. Yours is too.

Now apply both of these challenges to a chimpanzee. I doubt one could tie a knot very well. Washoe the chimp we taught sign language had a vocabulary of only 450. Yet, I have I on good authority that a very close relative of chimpanzees have developed complex thought, complex language and advanced tool use and creation.