Briton Hector Pardoe "thought my eye had fallen out in the water" after receiving an elbow to the face during the the men's 10km marathon swim.
Pardoe had frantically asked lifeguards "my eye, my eye - is it okay?" after he was left unable to complete the race, which was won by Germany's Florian Wellbrock.
He had been forced to quit with just a couple of kilometres left after his goggles were taken off by a stray elbow. He later posted pictures of nasty cuts, swelling and bruising around his eye.
The 20-year-old could be seen holding a towel to his right eye in his post-race interview. Describing what happened, Pardoe said: "It wasn't really what I expected from the start. I started really fast from the front, really warm conditions that I'm not used to. I've never raced in such hot waters.
"I started to panic when I was behind, I could see that any chance of top five, top six was over. I was trying to secure it as much as I could, didn't get the top 10 finish. I did well to catch the group up, I was feeling okay, I got back into it.
"On my last lap, I took an elbow to the face - I thought I'd lost my eye. My goggles came off completely. I always hoped that if I got injured like that, I'd be able to finish the race - but my goggles fell off and I couldn't even get them.
"I couldn't see anything, I thought my eye had fallen out in the water. I went up to the lifeguards and said, 'My eye, my eye! Is it okay?' They weren't giving me a very decisive opinion and I had to get out after that.
"It's fine, they're just going to stitch it up for me back at the Olympic Village. It was bleeding everywhere."
Gold medallist Wellbrock pulled away on the final lap to win by a dominating 25.3 seconds, finishing in 1 hour, 48 minutes, 33.7 seconds. He also won a bronze in the 1,500m freestyle on the last day of swimming in the pool.
Silver went to Hungary's Kristof Rasovsky in 1:48.59.0, while Italy's Gregorio Paltrinieri picked up the bronze in 1:49.01.1.
The Italian was silver medallist in the 800m freestyle in the pool. Defending Olympic marathon champion Ferry Weertman of the Netherlands finished seventh, while American Jordan Wilimovsky was 10th.