First, scale up the boat x100, as has been suggested.You can scale it back down when it’s done. Then explode it down to raw geometry. At this point, groups just get in the way. Now split the boat in half and discard half. No point doubling your work.
Now triple-click to select all and right-click > Intersect selected. This should for the most part allow you to remove extraneous geometry from the interior of the model. Also remove the back face of the pontoon, which can be easily replaced, so you can get at all the stuff inside the pontoon. Extraneous geometry is anything inside the outer skin–lines, faces, whatever. Get rid of everything that is not an integral part of a single, continuous skin. Everything on the inside should be blue. Everything on the outside should be white.
Close all openings and group the half-hull. Entity Info should say Solid Group. Redraw, reconstruct, and simplify as necessary to get a hull with no gaps and no excess material. Once you get half the hull to be solid, it should be easy to add the other half.
Finally, copy the half hull (inside the group context), flip the new half, and assemble it with the first half. Put any finishing touches on edge visibility.