There are more than 8 cipher chests so it doesn’t matter if you forgot the one inside the Sea Serpent. You can’t get back to it, but luckily, you only need 8 cipher chests for trophies and 100% completion. There’s one small exception – a purple language cipher chest is found near the end of the story in an area that cannot be revisited. All collectibles are in areas that can be revisited.
Some areas get locked off after the story but those don’t contain collectibles.
