Translation:The sea is quite calm today, my son.
I used "fairly" for "ziemlich" and was told by Duo that it should be "rather." The solution above says "quite." I think "fairly" should be accepted, and I said so.
Depending on how they are said, ziemlich and ganz can change their meaning. From the sentence above we don't really know the intention, so I would also support fairly. There's rarely just one correct solution, just look at the list of possible translations here: https://www.dict.cc/?s=ziemlich and here https://www.dict.cc/?s=ganz . There is at least a hand full of possibilities to translate the examples mentioned.