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.
Click on the picture
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.