How to Get a Boat
The easiest way to get a boat is to complete a quest that gives you a rowboat. There are a few such quests and they are usually located close to some story quest point. One example is at Sandrock in Dewstone Plains.
However, it seems that these quests go away after you hit a certain level. In that case, you will need to make your own rowboat which is easy. You just need to get 10 lumber and find a carpentry workbench to make one. You can then spawn the boat as you do with your mounts when you are at a lake or at the sea.