I just got a boat dog about 3 weeks ago. Her name is Plumb. She is a purebred pug.
Pugs are slow, small, can't jump very high, and are deceptively heavy for their size, 18 pounds. They swim like a panicking rock. Pugs also are allergic to fleas and are prone to hot spots in their double coat. They don't shed very much, and sleep up to 18 hours a day. They are highly trainable, very loving, and extremely docile and non-violent. Our pug was abused before we got her, and she is already becoming a very good dog. Her personality is incredible for having been abused.
They snore a lot, and their tongues often stick out of their mouths. I would suggest one as a boat dog. Plumb has 2 bad knees, so she needs help up the particularly tricky companionway in my boat, but otherwise she does alright.
Oh yea, pugs run at 103 degrees Fahrenheit, so they make a great lap warmer. They get cold easily, and overheat easily. I would recommend shaving them in the summer, and putting a jacket on them in the winter. They have very little fat, but plenty of extra skin, and a very soft, thick, but only 1/8" deep coat.
They are great to put in your lap in the cockpit if you are a cruiser type.
That being said, my mom just got a rat terrier puppy, and it seems a bit smarter, if harder to train. They are also better at navigating companionway steps, and have shorter, less soft fur.
I am also a fan of Glen of Imaal terriers. They shed a lot, but are extremely intelligent, long lived, and kind.
I would not suggest a lab. They are great dogs, but they love to "retreive" anything in the water, which could get frustrating if there have been really high tides, and they need a lot of exercise and are large.
I am pretty sure schipperkes are yippy. I hate yippy dogs. Pugs are the opposite of yippy, they are calm and loving, not barkers. They work as guard dogs, but are not obnoxious about it.