A man who once nearly drowned has extreme thalassophobia and fear of the dark; in fact he conceptualizes the sea as The Dark, and he believes that it is 1) out to get him and 2) able to access all places devoid of light. He also seems to be recovering weird memories that cannot possibly be his own.
Turns out he did in fact drown and was absorbed by The Dark, or Darkness, or Twilight (back-translating from Polish translation here) - an eldritch abomination hivemind / sentient primordial ooze that is the global ocean. Sometimes it would "secrete" or "reconstitute" a previously absorbed victim to serve as its agent on land. Our protagonist forgot about his "purpose" for some reason, and tried to live a normal life, until his phobias became unmanageable and memories of other drowning victims began seeping into his mind. A friend of his (who I think narrates the story) believed that his phobia was just trauma from near-drowning, and tried to cure him by exposure, only to witness The Dark claim the poor guy by dissolving him into protoplasm.
Quote (back-translated from Polish): "It's not sea! It's The Dark!"
I read it over 20 years ago, possibly about 30 years ago, when I was in my teens. I am nearly certain the setting was an anglophone country, perhaps between 1950s and 1970s. The MC might have been a veteran, WW2 or Korea (unlikely Vietnam) and his drowning might have been in a naval battle, but then again those could have been other people's memories, or I am confabulating entirely right now - I read it decades ago.