This PrusaSlicer project applies modifiers to the original 3DBenchy.stl, with 100% infill for the aft hull and lightning infill elsewhere. The resulting model floats upright when printed in PLA. This version is under 8 grams, but some of my previous test prints were over 12 grams and just barely floated, so there are many solutions to this problem.

floating_benchy_prusaslicer_project.3mf has everything preconfigured, but it may mess with your printer settings.

Alternatively, you can follow these steps to rebuild the project from the provided STLs:

Open 3DBenchy.stl and configure print settings:

• Perimeters: 1
• Solid layers: Top 2, Bottom 2
• Extra Perimeters if needed: Yes
• Infill: 10% Lightning

Right click "3DBenchy.stl" > Add modifier > Load > dense_hull_modifier.stl
Right click "dense_hull_modifier.stl":

• Infill > Fill density: 100%
• Layers and Perimeters > Perimeters: 2

Right click "3DBenchy.stl" > Add modifier > Load > light_hull_modifier.stl
Right click "light_hull_modifier.stl":

• Layers and Perimeters > Perimeters: 2

Enable "Center of gravity" in the Preview window, and verify that the CoG is at the bottom-rear of Benchy's window opening, as shown in my screenshot.

2025 update: I replaced the cutting planes with modifier STLs, so this should be easier to replicate in other slicers.
(To visualize the vertical cross section, I used PrusaSlicer "Export Toolpaths as OBJ" + Blender Bisect tool + Microsoft 3D Viewer in Orthographic mode.)