The Mariners need to add an impact arm, but none appears to be headed to Seattle any time soon. If Seattle cannot attract that arm in free agency or acquire one via trade, they must get better somewhere else. So why not build the best offense you can?
The Miami Marlins are in the midst of a fire sale, and the Mariners have already gone to the well once, acquiring Dee Gordon in early December. Now, they should go back to the well, and put on the full court press for Christian Yelich.
Yelich is perhaps the best player available on the Marlins, and would not be cheap to acquire.