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?