The antonyms for the phrase "casting anchor" can vary depending on the context of its usage. However, some possible antonyms could be "weighing anchor," which means to lift the anchor and prepare to leave; "floating," which is to stay adrift without being anchored; or "drifting," which means to move along without any clear direction or purpose.
In a broader sense, antonyms could also include "moving forward" or "making progress," as casting anchor implies a stationary, fixed position. Alternatively, "exploring," "journeying," or "venturing" could also serve as antonyms, representing the opposite of remaining stationary in one spot.