As to why someone is "rowing a sailboat", it could be that there are two vessels. The vessel with the sails is a ship (not a boat) and Michael is rowing the ship's boat, (dinghy, cutter, whatever) ashore. Sister is left aboard the ship. "Boat" in maritime terms usually means a small watercraft, and the big one is a ship.

On the other hand, on the Great Lakes one of those huge ore carriers is not a ship but a boat, for some reason. The same floating object on the sea would be a ship.

And just to make things more confusing, a submarine, even though its home is the sea, is "a boat". Don't ask me why.

DAve Oesterreich