My least favorite foods have all been from the sea. I dislike most but not all seafood to begin with, so this is just me picking the worst of the worst.
I'd have to say the #1 worst was raw squid hood. I was on a business trip to Japan for a week, eating horrible seafood after horrible seafood. Finally one platter of swimming nasty things came with what I thought was a carved radish (maybe I had a bit too much sake to wash down the rotted fish slime from the previous course). I gladly popped it into my mouth before realizing what it was. No, it was not a beautiful carved radish, it was raw squid hood rolled up to look like a carved radish. It was very slimy, had the taste and smell of a fish gut dumpster outside a fish cleaning station in the blistering heat of summer. But, what was worse was that it was like a leather belt. My teeth couldn't chew through it. But, as this was an important business trip, I couldn't just spit it out. I sat there on the floor in a cramped position for at about 10 minutes in pure misery until I could barely shred it enough, bit by bit, to swallow it all.
Next worst was whale. No idea which kind, they just said it was a small whale. It looked and tasted like slicing and eating a firm human crap.