Simple, native tools operation. Set Camera to Parallel Projection, select a standard view (I choose the Right view), turn on Hidden Geometry, edit the group and with a right to left selection select the faces that cross the horizontal plane. Right click and use Intersect Faces>With Model, then erase everything below the waterline. This method limits the number of faces SketchUp has to look at for the intersections so it goes faster.
[image: R24X_cut.gif]
By the way, your model has a lot of reversed faces. You should correct them. Only white front faces should be visible in your 3D model.
Someone will likely come along and suggest Zorro2 which can be a good way to do this but knowing the method I show is useful.