This is what it looked like when I flipped it over (next image). It was ~11 inches long.
It was a little wooden boat!
It was puzzling to wonder where it had come from. All of the parts were wooden except for four metal hooks on the upper deck (one was missing) and a few small metal nails or tacks along the keel. It looked like there used to be a mast in the center. The boat had been painted, and at some point it had been burned (see charred markings).
I was also very curious about the animals that were aboard. They were small, but if you looked closely you could see quite a few animals living in the grooves of the boat:
Because of their size, I decided to look at the animals under a microscope. It was a whole different world, and after doing a mini-survey, this is what I found:
Two species of acorn barnacles (larger one on the left with orange on the inside of the opercular plates, and smaller ones on the right).
One species of gooseneck barnacle (possibly Pollicipes sp.?) — it's the small stalked barnacle at the upper edge of the photo.
A spirorbid tube worm (white tube between the barnacles).
A teeny-tiny brown limpet (very magnified in the image above).
Long byssal threads that indicate mussels used to live on the boat, too.
And finally, a mystery critter! My first thought was bryozoan, but then I questioned that and wondered about tunicate (or sea squirt). I haven't seen many of them this small. If you're more familiar with them at this size, perhaps you can help with an identification? Below is another view, showing that this animal is stalked and branched.
It was an intriguing assortment of life aboard a little wooden boat!