crestling is a crest.
'Crestling' remains undefined in my dictionary.
The phrase "its beams effulgent on each crestling wave" describes the way light is shining brilliantly on every small crest or peak of the waves, reflecting a dazzling and radiant appearance on the water's surface.
To give a status. Origin: To give a kingdom or empire its Crest. The crest is the identity of the kingdom and the symbol of prestige, class and distinction.