Broken Shells

[Leanne]

Kudos for continuing your metaphor throughout and not going off track.  The problem I have is with the premise of the opening simile (and then on to the entire poem): this is not representing "life" in general, which is simply a matter of energy as any physicist will tell you, but a particular life.  Additionally, when we refer to a "seashell", we're talking about only the shell part so by using the term you're implying that its inhabitant has already died.  You mean a shellfish.  You don't actually need to beat people over the head with your setup, because from the rest of the poem it's pretty obvious what you're talking about.  You could just as easily start with "The shellfish begins life whole, purposeful..."

Life is like a seashell
Whole, purposeful
Until one day part of it dies
And it spends the rest of its existence
Constantly battling the tide
Being beaten again and again by the waves -- "again and again" is pointless repetition.  You could try just "battered by waves" or something similar.  Use a word that implies "again and again" without actually stating it.
Until it emerges on the shore
Finally tangible -- surely it was always tangible, even under the ocean?  And being broken has no bearing on its tangibility, so there's no point to a "but".
But broken