Ar var alda = “In the beginning was the Wave” is a very controversial interpretation that is not found in other translations. My interpretation is based in the understanding that ár is the ON adverb ór which means “in the beginning”. It follows that var (“was”) refer to the singular noun which was in the beginning, namely alda, which, when a singular noun, means “wave” (feminine singular). Traditionally, the sentence is taken to mean “early in the old times” or “early in the beginning”, understanding alda as a form of alðr, meaning “age”. My interpretation is, however, literal, grammatically correct and also entirely probable.
Maria Kvilhaug in The Poetic Edda p. 6 (note 5)