Recently, cross-medium communication combining acoustic wave and millimeter wave has gained more and more attention because it provides a direct communication method from underwater to above water without the need for relays. Presently, the feasibility of this method in short-distance communication has been verified. However, when extending to long-distance communication, the reduction in signal-to-noise ratio (SNR) due to the attenuation of sound and electromagnetic waves will present a challenge to the performance of communication. This paper proposes a packet-repetition-based scheme to achieve time diversity. Experimental results demonstrate that the pro-posed scheme effectively reduces the average bit error rate in translational acoustic-radio frequency channels.