On the water, the audience already understands what can go wrong. You don’t have to explain the concepts of “sinking” and “drowning”. The audience, in fact, already has an innate terror of these dangers, just waiting for you to tap into it. But in space, nobody knows the rules. Before your story really starts, you need to explain to the audience everything that can possibly go wrong on this imaginary spaceship you just made up, and some possible solutions as well. As soon as you’re done explaining them, anyone can guess what will happen next.
This is a huge problem for sci-fi and fantasy, but also for any setting where the rules are arcane: submarines, nuclear reactors, stock exchanges… If you describe the possible solutions before you need them, then they’re predictable. If you don’t, then it looks like you just made them up. You’re screwed either way. The simplest solution is to stick to boats, or other places where you don’t have to explain the possible dangers, but if you want to pick a more ambitious setting, then you have to be tricky.
Inception spent a whole hour explaining all of its complicated rules, which was way too long. But once they’d slogged through all of that exposition, they were able to deliver a fairly thrilling second hour (and a half). They were even able to “surprise” us in ways that didn’t feel like a cheat.
They kept stressing how crazy it would be to go three-deep into dreams within dreams, until we understood the concept pretty well. Then, uh-oh, our heroes fail at the three-deep level. Suddenly they get a wild notion: let’s go…FOUR-DEEP! And the audience gasps. No one had mentioned that possibility beforehand, but we instantly understood what it would mean and what a big deal it would be. The endless exposition had paid off: we understood this crazy world well enough that we knew what the rules were and how to break them.