You would gain Thassa, God of the Sea until the end of turn. It would cease being a creature but you would still control it and would still be able to use it's activated ability. Harness by Force only cares that it was a creature when you originally targeted it.
June 6, 2014 6:36 p.m.
Note that control of Thassa, God of the Sea will return to your opponent at the end of the turn. The permanent doesn't need to be a creature to return.