There is an excellent book that covers many of your questions: "Doggy on Deck", by Jessica Stone.
Doggy On Deck - Penchant Press - Doggie On Deck She does a very nice job of covering these any many other questions.
Although our dog is similar size, we have sort of the opposite problem: he never met a body of water, large or small, that he didn't have to swim in. We looked at various dog PFD's and found many of them to be inadequate. We ended up with one that has flotation along the sides and underneath his rib cage/belly, as this served to keep his head up higher. Also, when helping him out with the handle, these provide a bit of padding in the same area, so that he breathes easier, rather than having just the narrow straps underneath that tended to be constricting.
Is it a good idea to take your pup cruising? We don't know, yet. We're about to begin. Ask again, next year! After 16 years of doing it, though, through both the Caribbean and Pacific, Jessica got her answer -- absolutely!
ID