I KNEW IT!
I'd been getting suspicious about the initial voyage 'accidentally' ending up right where the sacrifice could be meaningfully made but this is one more piece of evidence that it's all been orchestrated. It's not a coincidence that the king has the same sea goddess' blessing. Calling it now, Mira (and possibly others) steered the ship towards her domain and from the start Mira intended to birth Diana into this world with the heart, maybe even making her the recipient of the blessing instead of Yulian. Or maybe there was another on the ship who was intended to receive the blessing and Mira simply improvised. In any case I'm more convinced than ever that Mira planned to obtain the sea goddess's blessing using the ship and the souls on board.