I literally devoured this book - finishing it in a sitting - which, regrettably, is the main problem with the book - it simply isn't long enough or detailed enough or broad enough, and, by concentrating on the Franks and Anglo-Saxons, to the exclusion of other, early Germanic tribes (such as the Goths, whose naval attacks on the Roman Empire in the 3rd C. AD/CE contributed hugely to the Empire's decline, or the Vandals, whose naval activities kept that declining Empire on the defensive), it emphasizes the huge gap in the historical literature covering seafaring between the Battle of Actium and the Battle of Lepanto. Nonetheless, limited as it is in scope, it fills, at least in part, a significant gap in the history of the sea and seagoing. One hopes that this book will finally open up the subject to further studies and treatises.
Regrettably, as well, the available evidence (both literary and archaeological) on Dark Age/Migration Era Germanic naval activities is terribly sparse, obliging the author to make numerous "leaps of faith", based as much on conjecture as on the evidence. Yet, in every case, his surmises seem reasonable and consistent with what evidence is available (indeed, one is rather reminded of Robert Bakker's "Dinosaur Heresies"; Bakker, likewise, indulged in - and was highly criticized for - a lot of extrapolation and "creative" interpretation of the record, in coming to his conclusions, however, outrageous as his opinions seemed at the time, the subsequent 30 years of paleontological efforts have confirmed most of his surmises and, in retrospect, his predictions and interpretations seem absolutely prescient). The "central thesis" of this book, quite simply stated, is that it is highly unlikely that the established, even thriving culture of Late Roman and Sub-Roman Britain was overthrown and supplanted by small groups of people coming across the North Sea in tiny row boats with minimal freeboard (hardly a radical notion, tho' it certainly does conflict with the "conventional wisdom", which would deny the Franks and Anglo-Saxons even the knowledge of sails and wind power, let alone their utilization of it). That "radical" notion (and its implications for ship building) is well supported, so far as the record allows, and the book does a good job of marshalling that limited information in support of its conclusions. Nonetheless, there are likely many who will continue to deny that the early Germanics were able to make and operate the kinds of vessels which their manifest accomplishments suggest that they must have possessed.
What the subject needs is a major work, addressing the seafaring activities of all of the early Germanic groups. This book is not that work, however, it is a "down payment" on it and, as such, should be a welcome addition to the library of anyone interested in the history of man's activities on the sea.