Step 1: Concept
Power for a flowing fluid is proportional to the cube of the velocity ($P \propto v^3$).
Step 2: Meaning
Doubling the volume of water in the same time means doubling the flow rate, which requires doubling the velocity ($v' = 2v$).
Step 3: Analysis
Since $P \propto v^3$, the new power $P' \propto (2v)^3 = 8v^3$.
Step 4: Conclusion
The power must be increased by a factor of 8.
Final Answer: (B)