One example might be the Greek geographer Pytheas, depending on how one interprets other writers' citations of his work. Pytheas travelled to and wrote about a far northern land he called Thule and mentioned that further north air and sea were mixed, which might be a description of the Arctic Ocean. This is discussed in some earlier threads on here, like this one by u/KiwiHellenist and this by u/Cats_and_Museums. I should note that it is possible that Pytheas had seen ice before since the temperature can sink to negative degrees Celsius in his hometown of Marseilles (Massalia in his time), but the frozen sea must have been quite different from even a cold winter day on the Mediterranean coast