From the OED entry on "Knell":
"The sound made by a bell when struck or rung, esp. the sound of a bell rung slowly and solemnly, as immediately after a death or at a funeral."
By stating that sea-nymphs ring a knell for Alonso, Ariel paints his death as the death of an important man, worthy of being broadcast to the larger population that they might mourn. This line of poetry also associates the knell with the hourly motions of the sea, or the tides, suggesting an even greater omnipresence of the funeral dirge: every wave becomes a part of Alonso's procession, honoring the loss.
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