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.