Dead body's do float but they wouldn't for 100 miles there body's would absorb the water and sink at some point and I don't think zombies are the best swimmers to make it out to him fast enough.

Being on a boat would be a bad idea anyways unless u had tons of bait and fishing supplies there would be no food source and if you could manage to catch fish for that long u would run out of wood, gas, ext to cook them. At some point you would need to come back to the mainland or an island.