I had sea urchin roe (uni) sushi one time. I always saw them using it on Iron Chef and had to try it. It was slimy and pretty nasty.
The one I was amazed by is eel. I've tried that a few times, and it's slightly sweet.
Eel is very delicious. Seal is pretty tasty as well.
I forgot about seal. I had raw dried seal at a powwow once. It was...different. I've also eaten a buffalo often (a cultural thing). It pretty much tastes like beef.