Hycean worlds are planets which bridge the gap between rocky and gas giant planets. They have atmospheres high in hydrogen and vast oceans with perhaps no dry land at all. How could the local sentient life develop space flight? My only idea is they have jellyfish/coral organisms which could metabolize hydrogen to float which they could engineer to form ships, but that feels like cheating.
It might be impossible
Sorry to say it, but it's likely impossible. Hydrodgen-based floating obviously doesn't work outside of the atmosphere, and without dry land, good luck using metallurgy ─ or even fire. It's possible that a water world could develop sentient beings, but not ones with much technology.