Andre Norton's sci-fi tales are just as enjoyable as when I read them over 50 years ago.
This book was first published in 1957. It is a tale of an American teenager who lives on an isolated West Indies island with his father, who is researching a "red tide" killing fish. The boy misses the states and want to leave the island, but has to stay with his father. He does a lot of scuba diving, helping dad out, and is friendly with the native islanders, even the local "witch doctor."
Suddenly, the U.S. Navy moves in, with permission from the British government, and sets up a secret base---no islanders allowed, not even the American scientists.
There is evidence of strange sea life in the area, with accompanying radioactivity. Meanwhile, political tensions between nations are bringing the threat of atomic war closer. The Navy, scientists and islanders clash. Then inter-island boats either go missing or are found without the crews. Are they being kidnapped by the Russians, the U.S. Navy or something else from the sea.....?
That is the summary for the first half of the book. I won't spoil it for you by summarizing the second half.