Olympian Katie Grimes Admits She Was 'Nervous' To Swim In Dirty Seine River

Katie Grimes admittedly had some things other than her opponents on her mind before hopping into the Seine River at the Olympics earlier this month ... telling TMZ Sports she was actually "nervous" about the water's cleanliness.

Of course, the E. Coli levels of Paris' famed body of water were a huge talking point in the leadup to the Games ... as the city dumped a fortune into making sure it was safe enough for athletes to swim in during the Olympiad.

On race days, officials deemed it was all OK for competitors ... but Grimes says when it was her time to dive in at the Aug. 7 marathon event, it was on her mind.

Thankfully, the 18-year-old said once she got in, she "didn't think about it once." And, more importantly, she said it didn't make her sick.

She ended up finishing in 15th place in the event.

Others seemed to have similar experiences to Grimes -- as even though there were some upchucking issues , they didn't appear to be related to pollution.

As for the rest of her time in Paris, Grimes -- who was working a ceremonial shift at a Raising Cane's this week in celebration of her appearance in the Olympics -- earned a silver medal in the 400-meter medley, and she told us it was truly "special" to earn the award.