If you really want to display a dive certification patch, my vote would be to sew a SINGLE patch onto either your regulator or boat bags, or perhaps a backpack. If you went that route, I'd sew on something random like "Underwater Naturalist, or Diver Propulsion Vehicle Diver". I'm still a new diver (MSD), but the lesson that's plainly clear is that there's always a more experienced diver around, so why not have a little fun opposed to being a braggart advertising every accomplishment.
I've gotta say the thing I always preferred about the Corps was the lack of unit patches, or name tapes for that fact. I dreaded the day I had to sew name tapes on my cammies.
I thought every Marine had a tattoo, either the EGA, bulldog or siliy-stamp. I've got the silly-stamp (USMC) on my upper arm. It always seems to start conversations on the dive boat as there are heaps of vets who dive.