I've been looking forward to reading this book for some time, and finally started on my trip to Houston just before our Alaska trip, then finished it just now in the Wheelhouse Lounge on the Island Princess as it entered Glacier Bay.
Back to the book. Keegan illustrates the history of war at sea (he never really explains the title) using four battles, Trafalgar, Jutland, Midway, and the Battle of the Atlantic (specifically two convoys' struggles against U-boats in March 1943), and ends with a quick conclusion that the submarine is the naval weapon of the future.
His summary of the background on navies - originally created to protect a nation's trade fleets from pirates, and only later used to assert national primacy over a conmperting nation's fleet of warships - is a good and something I didn't know. His lead-in to Trafalgar and analysis of the battle itself was very good, especially since I knew virtually nothing about the battle itself (after all, Hornblower wasn't there). The chapter on the Battle of Jutland was also quite good, although Massie's analysis of it in Castles of Steel (the followup to one of my all-time favorites Dreadnought). But when he got to Midway, factual errors started creeping in. There was the almost standard provision of a photograph of the wrong Yorktown, and while he narrowly got right that the Army B-17s that attacked Nagumo's fleet did no damage, he mistakenly asserts that the U.S submarine Nautilus sank the damaged carrier Soryu, something no historian I'm aware of in the last forty years has claimed (Price of Admiralty came out in 1989). While it isn't surprising that his analysis doesn't have the insights that Shattered Sword provided (although he tracks parts of what their analysis that I've never seen anyone else do) the analysis of Midway is still a bit off, and inexplicably his bibliography doesn't cite Walter Lord's seminal Incredible Victory, which came out over 20 years before. Granted that's my all-time favorite work of nonfiction so I'm perhaps a bit biased, but how he could publish a book on Midway and not even cite it as a source is inexplicable to me. He could disagree with it - Shattered Sword certainly did and I agree with it, but not even citing it perhaps led to the Nautilus/Soryu errors, as well as others.
Finally, the chapter on the U-boat battles is just narrative, and while well-told, it was just a snapshot of the seesaw battle that went on from 1939 to 1945. And the hasty postscript that "submarines are the ultimate weapon" seems almost as dated as the prior assumptions that battleships and carriers were. While I can't say I disagree that subs are a necessary part of any navy - assuming you don't have the national resources to operate a carrier fleet as well - I think that both carriers, subs and surface fleets have benefits, and which is better depends on who you are and what you have to contend with. In short, your options are a lot better if you have a five billion dollar carrier task force hanging around (which necessarily requires another two task forces in bases refitting for their turn, and several other trios in other parts of the world). I am probably judging the book with hindsight because Keegan was writing during the Cold Ward, when a substantial Soviet fleet was opposed to the U.S. Navy, and the problem was far more acute then than now, when there is in effect no other naval power that could compete with the U.S.
Anyway, I did enjoy the book and would recommend it. I just think the premise is a bit dated, and the Midway section (which is all I have outside expertise on) has a few errors.
In the end, I think the book might better have ended with Jutland and contrasted it with Trafalgar, because that was to me the best analysis the book had.