stayed > stopped; prevented
4 From drowning. But more happy he, than wise:
happy > lucky
5 Of that sea's nature did him not advise.
Of that sea's nature did him not advise > [Atin did not consider the nature of that sea]
6 The waves thereof so slow and sluggish were, 7 Engrossed with mud, which did them foul agrise,
Engrossed > {Rendered gross, dense, thick} them > [Atin and Pyrochles] agrise > horrify
8 That every weighty thing they did upbear,
That > [So that] they > [the waves]
9 Nor aught might ever sink down to the bottom there.