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.
Doflamingo is well aware of how Law's powers work, and he made sure for most of their interactions that Law coulst set uo his Room by keeping him on the defensive. And along with that, he'd need to be near the water first for that to happen and Doflamingo probably wouldn't let that happen.
Also, DF powers don't instantly become inactive when they, or the user touch sea water. This was proven when Aokiji touched the ocean physically, then froze it for miles across when first meeting the Straw Hats.