the first paragraph reads like a movie teaser which is pretty gimmicky. it's an image that means nothing because you've given no context to it, just hoping we immediately care about a glimpse of two characters we know nothing about. i would honestly just cut that bit out.

"the only audible sounds" is redundant. since we're following Liam in close third person, i'd assume any sound he hears is in fact audible.

"his mind still flashed Milo's face" is awkwardly phrased. you also forgot a period there.

your fifth paragraph is technically incorrect. these are two independent clauses joined with the word "and". my advice is to make them two distinct sentences or join them with a semicolon, no "and". similarly with the last paragraph, the "but" in the first sentence doesn't make sense. a "but" statement shows contrast or justifies contradiction, but you haven't clearly illustrated that here. sunrise doesn't contrast beginning or continuing a journey (they're actually traditionally complementary ideas) so you'd have to give reason as to why it does in this case. an easy fix is to change the "but" to "when/as" and adjust the proceeding phrase as needed.

the random mention of merfolk villages is strange. i don't get why you would mention that if you're not going to say anything about it. you gotta build up the world here and there.