ISLANDS IN THE STREAM by Ernest Hemingway. 466 pages. Scribners. $10.
Those who forged through last year's biography by Carlos Baker may recall that Ernest Hemingway, in the late 1940s and early 1950s, was engaged in writing three loosely linked narratives. Somewhat Delphically, he referred to them as The Sea When Young, The Sea When Absent and The Sea in Being. The first two apparently dealt with a famous painter named Thomas Hudson enjoying a Bahama vacation with his teen-age sons and then, later, hunting German submarines around the Caribbean in his fishing boat...