Researchers at the UMASS Amherst Fluid-Structure Interactions Lab have built a fish that can achieve a speed of 4 g from a standstill. Some fish, like a Pike, can achieve speeds of 15 g for a few seconds. The researchers state:
We have built a simple mechanical system to emulate the fast-start performance of fish. The system consists of a thin metal beam covered by a urethane rubber, the fish body and an appropriately shaped tail. The body form of the mechanical fish was modeled after a pike species and selected because it is a widely-studied fast-start specialist.
We have seen other robotic fish, such as the one from Maurizio Porfiri, but this fish from UMASS Amherst is so far the fastest robotic fish and will someday be able to navigate rough seas or escape imminent danger.