Yeah I can imagine a jam-packed cruise ship might be useful provided the signal propagates from deck to another (unlikely), but it's quite a niché use case.

>Also why would one want to have the data go over some servers thousands miles away when the device is right next to you?

Why would that matter? Use Signal to protect the content, or use Cwtch to protect content and metadata. If you need to exchange secret communications that mustn't go through some server, why not discuss f2f with no phones around? You'd also eliminate attack vectors where your (chances are, Chinese Android) device spies on you, as well as anyone who has compromised it to read messages from screen.