Why is Law unable to use Shambles against Doflamingo and simply drown him in the ocean? He could've simply replaced a fish in the ocean for Doflamingo. Unless I've missed something.
If I am not wrong, Shambles is nothing but a technique which interchanges positions of two objects! DF can operate on almost anything in the world, but as we know its weakness is sea and sea related things like Sea Stone. Your power becomes null when you interact with the sea. Pushing Doflamingo into the sea will require pulling out some sea water and exchanging its position with Doffy, which clearly is not possible. Think about it: the guy Señor Pink has the ability to swim anywhere and the irony is he can't swim in the sea.