"The Atlanic is cold."
Translation:Mae môr Iwerydd yn oer.
Thanks for this comment. A little bit of a puzzle here. Firstly this phrase was deleted since it was felt that 'Môr yr Iwerydd' (lit sea of the Irish) was the correct translation for the Atlantic Ocean. However it looks like 'Môr Iwerydd' is also considered an alternative.
In terms of your sentence, if 'Môr Iwerydd' is acceptable as Atlantic Ocean then your sentence is completely correct and the course is wrong.