The literary techique used here is a kenning.
A kenning is a miniature riddle that usually had two parts and was frequently hyphenated. An example of a kenning from Beowulf was "dread-loom." The Anglo-Saxons used this term to refer to God's plan, making a comparison beween weaving and the awe inspired by God's plan.
"Wolf-of-the-waves" is used here to describe Grendel's mother, a beast who lived underwater.
