After exiting my boat, I broke it in order to let out my tamed cat. When I broke the boat, the game froze for a split second, the boat broke, and my cat disappeared. I saw bubbles in the water, as if it had fallen in and was swimming, but there was no cat to be found.

I tried to exit the game without saving to see if I could restore an autosave before the cat disappeared, but when I returned to the game, I was a short ways back in the ocean where I'd come from, as to be expected, but both my cat and the boat were gone.