Line 420 may be a combination of various allusions. Here are some possibilities, not all are recognized by authorities.
In William Shakespeare's play Coriolanus the general Caius Marcius, called Coriolanus, (based on the historical Coriolanus, the general Gnaeus Marcius) has been exiled from Rome and is leaving the city. He mentions to friends and family "That common chances common men could bear; that when the sea was calm all boats alike show'd mastership in floating" indicating that it is times of stress that shows a man's spirit. This quote appears at the very beginning of act 4, scene 1.
Coriolanus' name appears in The Waste Land just a few lines earlier (at line 416).
The poem Dover Beach by Matthew Arnold starts with the line "The sea is calm to-night."
Eliot wrote a critical essay on Matthew Arnold.
John Milton's poem Lycidas has in one of its lines "The Ayr was calm" which is not the same as "The sea was calm" but it is an elegy for a friend who died at sea in a shipwreck. There are other reasons to examine this poem.