The Man Who Fell to the Garden - The Ohoopee River Anthology, #3

David Langston is a man who suffers from a life-threatening ailment brought on by complications from an mRNA vaccine and the Covid virus, complicated by a pre-existing injury. Living alone in a quiet English cottage, cared for by a spunky housekeeper who sees David as her personal project, David's life abruptly changes for the better when an odd stranger, Arthur Gibson, suddenly appears in his garden one day with the fantastic tale that he was a survivor of the Titanic, and that he was rescued by mermaids.
He offers David a cure in the form of a ring made from pure Iridium. The ring works, David is cured, but complications arise, and the ailment returns, only worse than before. Two research scientists, a mad prophet, and a host of very mysterious characters join Arthur in the race to find a permanent cure for David before the disease claims his life. The search for the cure takes David on a journey that ultimately leads him to a secret research center on the ocean's floor where no one he meets seems to be willing to give him honest answers about this peculiar facility and its inhabitants.