In this paper, we present the development of a multiple robotic fish cooperation system - Robofish Water Polo, which is built on the basis of several multi-link biomimetic fish-like robots designed in our. The motivation of this work is that the capability of one single fish robot is often limited while there are many complex missions which should be accomplished by effective cooperation of multiple fish robots. Robofish Water Polo, as a novel platform of multiple robotic fish, can be applied to test and verify different algorithms and strategies for cooperation of multiple underwater robots. We have organized a two versus two competition to demonstrate the performance of this platform.