A Dock Master will allow you to travel by boat around the realm, when you right click the NPC you will be presented with a shop window with a list of all the tickets that are sold. Buy a ticket from the NPC (they are free) and hand it back to him and a boat will be summoned at the end of the dock. You have 30 seconds to board the boat by double clicking, right clicking or pressing g when near the boat.
You can dismount a taxi boat at any time by pressing the space key.
Multiple people can ride in the same boat, so if one is summoned at the dock already you can board it.