I'm thinking a girdle furled is it being folded about and wraps around you and he's describing the ocean in this manner suggests something to do with the oceans wrapping around continents,ect. of the world, how we're protected in a sense. In full context of the poem though it seems the author is talking about god and faith as in "sea of faith" and how faith was once prominent surrounding all things and the bright girdle was to show how prominent it once was. good luck with english!
