My power went out in the middle of writing my review for you, so sorry if this review is terse, but I'm trying not to just say, "forget it!"
It sounds neat to start. Being a Christian on a worship team this beginning melody reminds me of some of the melodies we use in CCM.
At 00:47 I suggest changing the xylophone's melody line to a different instrument, like the washed out oboe that has a trumpetesque timbre your using that literally just goes away at that point and would be a nice layer insrument for that section regardless. It sounds really unnatural to keep this little plinky thing going while the string section is swelling like that. Another option would be to increase velocity of the xylophone there to make it sound more naturally orchestrated. I get that you're going for serene underwater, but- oh it just hit me that at the end of your song you bring in a piano, you could give the piano better incorporation by making the xylophone bit the piano during the loud parts, cuz the piano is pretty random.
So anyway at 01:22 I would go back to using xylophone and then at 02:07 I would either switch it to piano or have the piano and the xylophone play the same melody layered over each other. As I think of it I would go with the former.
I like your Grant Kirkhope-esque waltz you threw in at 01:32.
I dislike how the song ends and would prefer an ethereal wash to end the song rather than a swell and some weird cadences. The thing at 02:43-02:50 sounds like I'm starting up Fallout 3 since it is the same melody and chords. I'm not saying your ending is uninteresting, I just don't like it and am not so sure it lends a lot to the composition.
To conclude, I generally like the song, but there are things that are clawing at me about it. The instrumentation and arpeggios of the song have reminded me of the FF7 OST, specifically 'Anxious Heart' and 'Tifa's Theme' (not that the song is a lot like these, it just reminds me of them in a way).