To place a boat, drop it from your inventory onto a water surface.
You can board the boat by typing /vboard. You can leave the boat by jumping, the same way you do with a horse.
You can invite people on to your boat by using the /vinvite command.
Once you and your companions are on the boat, you can move by clicking on the water surface where you want to go. You can also follow someone else with the /vfollow command. If the water is not deep enough where you are going, the boat will stop. You'll have to maneuver around the shallow area.
Boats will travel more slowly if there are not sufficient people aboard. Until you reach one-half of it's maximum capacity, the boat's movement rate will be proportionally decreased. In addition, you need at least 1/4th of the boat's maximum capacity before the boat will move at all.