Birth Day

I think you have asked very good questions here!

First I do think the poem transmits meaning to a larger audience, in a more mysterious way, probably and that is a good thing.

I don't necessarily I think the present tense makes a poem stronger, sometimes it does...it depends. This poem has the feeling of a dream, a memory, and as the symbols seem "mythological" the past tense in the first part seems appropriate to me. The change to the present is as if awakening, and I like that.

I think I like  more of a break after "bear witness" transitioning to another tense... it makes it more dramatic as if stepping to the edge...of the poem.

The centering... I usually prefer not... with this I am less adamant about that... but probably prefer it without.  I would perhaps go through and take out a few "extra" words, but I would be careful to preserve your intended lyricism... the music of the sea, sound of waves is important here, I think.