Grammar question. A queue of divers lines/line up? I seem to have trouble understanding which is the subject in sentences like this one.?

Grammatically the sentence is wrong to begin with. "A queue" (as they say in Britain, or "a line" as we say in the U.S.), can't "line up". The divers can line up to form a queue, but the queue itself cannot line up. The grammatical error could be what's throwing you off, so first let's correct that.

You could say "A queue of divers forms". Is it easier to see the subject there? It's "a queue", and a queue forms.

Or you could say "Divers form a queue" or "Divers line up to form a queue". Here "divers" is the subject, so it's "divers form" or "divers line up".

The queue is the subject - and it's one queue, so it's singular.

All I would say is that a queue cannot line up. It IS a line.
That would be like saying that 'a line lines up.'
'Many divers line up' or maybe 'a queue appears/forms'.