I'm not an expert, but I think this is an incorrect assumption. We can navigate it decently on the surface (even then, plenty of ships sink/go missing each year [1]) but underwater is a whole different story.

> Why is this vessel unable to stay in contact with its parent

That is kind of the big question here.

[1] https://en.wikipedia.org/wiki/List_of_shipwrecks_in_2022