I don't think space is milder, but to get up there, you first have to build a working rocket, which will make sure that you generally know what you are doing. The ocean doesn't have such a barrier, you can theoretically lower anything into the water and try to get to the Titanic...
You can’t always get human contact underwater. If you can’t come up to periscope depth, comms are extremely limited. Even when you can, the response is likely to be “fix it yourself.”
It was a strange and isolating feeling the first time something broke underway, and I realized there wasn’t someone to ask how to fix it. It’s also a great builder of grit and determination, or of hopelessness. People tend to go one way or the other.
Being awakened with a flooding alarm when drills were not scheduled, for example – I realized afterwards (as did everyone else, I assume) that if faced with an actual emergency, I could keep my cool and work the problem. [0] It’s a good feeling, especially when you know you can trust the other 100+ people around you to do the same. Incidentally, it was not technically flooding (unplanned water ingestion at a rate greater than the capacity of dewatering systems), but sure as hell looked scary when they demonstrated the flow rate seen later.
[0]: I personally think this is why Navy Nukes (especially submariners) make great SREs. The fact that a Nuke helped build Google’s SRE program probably helps.