If you haven’t done so already, please read the Wiki. It’s important to to have a good understanding how the different Buoyancy Modes work.
You’re right, SinglePoint Mode will not look good for a boat of that size. It takes the translation and rotation directlyl from a single point on the water surface. The advantage is that in this mode the boat can’t become unstable.
In AutoPoints mode, Simulate Physics is turned on and a physics simulation with your boat is performed. Please check if your boat has a realistic mass, and that the center of gravity is where it’s supposed to be. If for some reason it is too high or too low, the buoyancy won’t work correctly. Next, please enable the DrawDebug option to see where the Buoyancy Points are placed. On the example boat, you can see what it should look like. If you can’t see the Buoyancy points, the problem might be that the Buoyancy blueprint failed for some reason to correctly determine the dimensions of your boat. There is also the possibility to place the Buoyancy Points manually with the ManualPoints option, but let’s first try to get in to work in AutoPoints mode before considering that.
It is also a good idea to check if your boat behaves correctly in a physics simulation, for example just drop it on a plane and observe if it moves in a realistic way.
If all this fails, I suggest that you send me an email to the support address and I will help you get this working. Also, you can send me a small example project with your boat and I will have a look what’s going wrong.