Aquatic Lifeforms on ocean planets

I had an idea for a great new episode. I would like Isaac Arthur to produce a video about his thoughts on what Aquatic Aliens would look like. Maybe elaborate and expand with other types of alien life and their forms. That would be a fantastic video to watch. I've seen all of his alien predictions ( secret aliens etc.).
Thank you for your consideration on this idea.
Jimmthegrimm

That's a great question. It's been gnawing at me too.
The answer probably most depends upon the type of world we're looking at. Is it a water world with extreme depths? Never touching a hard surface would probably have an effect on the development of life. A water world with shallow seas would most likely development much different life. An ice covered water world would also developed different life.

That being said I get the feeling that the majority of life we ever detect on a water world with a reachable bottom is going to look and act a lot like the mud in your local pond. Single celled organisms with some more complex worms running around. That was life on earth for 2.9 out of the 3.5 billion years life existed. A less hospitable planet could see life stuck at that stage for much longer.

I've wanted to see an episode about the development space-faring technology from a marine civilization as well. There's an assumption that yo uneed to develop fire before you can progress, but what if they develop other means of chemically altering things? Such as a concentration of acidic, basic, salty, or fresh water? You could experiment with various concentrations of different substrates. Maybe even create fire within an oxygen bubble. It would have to be lined with some pretty strong mucus or soap though.

Also, if life lived near the deep underwater vents, they would have access to lots of chemical reactions to observe. I assume that most of the civilization would be on a reef like area with lots of things to do, rather than drifting through empty ocean going in -between places. But then again, long-distance communication is possible with whales, so maybe it's not so boring out there? You could share knowledge across huge distances that way.

There would have to be some way of handling tools or at least creating things. Maybe a nimble tentacle or two can break rocks and such. But create machines?

Imagine McGuyver born in the body of an octopus, what would he do?