I want to develop a water-dwelling, intelligent species that could ultimately reach space.1 My question here is about the biology of such creatures.
I want my creatures to live in the water, not move onto land, but shorter outings onto land not only would be ok but would help my plot. This makes me think somewhat of whales and dolphins, which breathe air, live in water, and cannot survive long on land (at least without special intervention).
But whales and dolphins don't have anything like hands, and tool use is an important step along the progression that leads to advanced technology. While some dolphins use tools, capacity without something like hands seems limited. (If I am suffering a failure of imagination, feel free to challenge this in an answer!) We know that amphibious creatures can evolve hand-like parts; none of the examples there are very advanced, so I don't know to what extent this "goes with" the other developments I need.
Access to land implies some mobility on land. I assume this means something like legs (with flippers?). My creatures are native to the water, so it makes sense to me that mobility on land would be harder, but it should be possible.
I'm not looking for magical approaches like selkies.
How should I approach the biology of my water-dwellers? What combinations of breathing methods, appendages for tool use, appendages for mobility, and developments that support intelligence are most plausible?
I am aware of this question, which asks how a deep-sea civilization would evolve in a particular environment. I'm willing to adapt my environment to fit my creatures, rather than going the other way around, so while that question is somewhat helpful to me, I don't think it's a duplicate. I'm also aware of questions like this one and this one, which are about technological development but not about biology.
1 For purposes of my story they don't need to develop spaceflight, just use it. The water-dwellers want to leave their planet because the land-dwellers have polluted their environment beyond repair. If they can steal a ship from the land-dwellers and adapt its environment for their needs, that's fine. But they need to be advanced enough to operate it.