The Priest is appealing to the king, Oedipus, on behalf of the people of Thebes.  He points out that people of all ages are at continually at the altars, praying to the gods for relief from their trials.  Then he says this:

You can see for yourself--the city is like a ship rolling dangerously; it has lost the power to right itself and raise its head out of the waves of death.

Thebes is clearly dying.  Crops aren't growing, herds of cattle are dying, babies are being born--stillborn.  The city is full of weeping and crying and mourning and death, and their prayers have been ineffectual.

The simile of the city being like a ship which is nearly on its side, taking on water, about to sink, is apt.  The only thing they know to do--pray--is not putting them back on an even keel.  The reason for that, of course, becomes clear throughout Oedipus.