I actually work in this space. The difficulties of long-running underwater probes should not be discounted. Comms bandwidth without a tether is… quite slow. Dealing with even the tiniest drops of water inside the system is… a real problem. Salt water is also quite a problem. Deploy and retrieve is a real problem.
I won’t say I think outer space is easier, but the problem space is very different.
I won’t say I think outer space is easier, but the problem space is very different.