I still hang on to my earliest understanding of the poem - yes, a fairy tale but then till the very late nineteenth century most poems were to be read as a window of truths and emotions - Romantic poetry - rather than as a literary story or explaining a happening or a political, social, and economic, concern that is more popular after Tennyson.
We read over and over how Tennyson was on the cusp fighting with the change in poetry that coincided with the Industrial Revolution. He preferred the older Romantic viewpoint for his work. And like any fairytale we know the characters are not only make believe but they usually represent something - To me the way to think of this is like when we see on TV or on paper a large muscled bald man in a white tee shirt, we know he is an image for a cleaning product - and even as children reading Red Riding Hood we knew a wolf does not talk or hide in grandma's night clothes after acting as a wild animal and eating the grandmother.
And so the Arthur stories, like so many myths of many cultures - have a meaning beyond the obvious story - as in her earlier post, Halcyon recollected a Native American myth - Some of the early myths, hundreds of years before Rome came to British Isles, these old stories are symbolic tales of cultural values, loyalty, sacrifice and death, redemption, bravery etc. etc.
Tucked in my reservoir are symbols and their meaning that others may not have had similar associations - all I can do is share what I see in this poem - my viewpoint is not right or wrong - it is not meant to take away from anyone's interpretation of the symbols Tennyson used in this myth poem.
It may help to understand my philosophical education was less of Thomas Aquinas and his duality between essence and existence and therefore, having to prove the existence of God but rather, more of Mister Eckhart who says, that in God there is an intelligence which precedes being, existence and understanding are the same and we, as rational creatures, are gifted with a spiritual soul. Both Thomas and Mister Eckhart were philosophers 200 years before Luther and are studied in schools today as thinkers from a common Christian philosophy.
In one of his many papers Mister Eckhart has a beautiful description of the souls connection to God - so wondrous is the description it sends chills up your spine. And in the spirit of that understanding this poem can be one of Salvation, Regeneration, Passing into the life of the spirit, Justified by faith, that is included in the Arthur myths.
The reapers reaping barley, the grain of bread that is the symbol for the Body of Christ.
She looks into the mirror, as the eyes are the window to the soul so, the mirror reflects back to us Justice. In the language of Scripture, Justice means not only that great moral virtue which gives everyone his due, but it represents a perfect state of soul, the perfection, the completion of all virtues.
And in this mirror she sees Lancelot - not the human sexual man but the symbol of the glorious container for God's virtues as were the characteristics of a knight. The way Tennyson describes him it is not hard to see the likeness to a Monstrance - that holds the host, the symbolic body of Christ, displayed during the holiest of ceremonies like on Holy Thursday.
A photo -
She, surrounded by the Lily, symbol of the Annunciation, goes to the water, in Celtic culture, access to the other world, in Christian culture, regeneration, Baptism, born of the Spirit. Enters a boat, the church, the ark, the ship of salvation also in Celtic lore, Manannan who sailed without sail or oars - here is a bit about the Celtic God Manannan
http://manannan.net/whois/index.htmlShe floats in a river, a rite of passage from one state to another, in song, the harmony of the sphere of life, sailing through a cold rainy night, the creative power in spiritual darkness, to her death, which symbolically death simply means death to the earthy life preceding the spiritual life, which is the core of our sacraments, Baptism, Holy Communion, and so forth according to if you are Anglican as Tennyson or seeing the symbolism as a Catholic.
As she floats past the walled garden, field of the souls, onto The kingdom of Camelot, the bright, idealized city which could only have been found in legend, where Lancelot, in his goodness says, "God in his mercy lend her grace." Which goes back the Eckhart who says, God gives us grace and that grace is what the soul rides on as a direct link to God.
There are many more bits and pieces that can be understood from this myth but those are the highlights to give the symbolic message of this poem that is beneath the Aquinas like existence of the storyline. I have so enjoyed this discussion because I did see the differences between Aquinas and Eckhart play out as this, yep another long one, essay speaks...
http://www.quodlibet.net/articles/drazenovich-eckhart.shtml 
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