The Brain is deeper than the sea— For—hold them—Blue to Blue—
The second stanza opens with yet another comparison. This time the speaker is telling us that the brain is deeper even than the ocean.
She mirrors the first stanza exactly by using the next line to set the point of her comparison.
Before we check what she has in store for us, can we just take a second to think about this "Blue to Blue" thing? Okay, the sea is blue, sure. But are brains blue? Aren't they like pink or grey?
We're guessing our speaker is getting all poetic on us here. It could be that she's using the color blue to represent anything that's super deep and seemingly unfathomable—the sea, the sky, your blue jeans, etc.
The one the other will absorb— As Sponges—Buckets—do—
The speaker sets out to prove her deep brain hypothesis by saying that the brain can absorb the whole ocean.
She uses a simile to compare the brain's amazing absorbing power to the way a sponge can absorb all the water in a bucket.
Of course, our speaker can't possibly mean all this literally. If you tried to absorb the whole ocean with your brain, you would have one seriously soggy brain afterwards.
Instead, it seems like the speaker is referring to the brain's ability to absorb information and to understand more and more of the world around it.