In this shot you'll see that some places were "solid" down to the headliner.. where some holes weren't.
When you try this.. you'll be frustrated that you can't get stainless bolts exactly the right length.
We cut many bolts.. becase we didn't want them below the headliner where possible.
Also... We used a 1in hole saw to cut all the holes in the headliner. Plugs were readily available at our local hardware store. Remember though! This will limit you to a 1in.
Fender washer. Is that enough? You decide.
Cutting a section of the headliner out and installing a backplate is also an option. Totally depends on the construction of your boat.
Don't put the plugs in for a couple of months though so you can inspect regularly for leaks...