We experimentally demonstrated high-speed diffuse line-of-sight optical wireless communication across a wavy water-air-interface. The testbed channel was evaluated, in terms of data rate, coverage and robustness to the dynamic wave movement, based on the performance of different modulation schemes, including non-return-to-zero on-off keying (NRZ-OOK) and quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM). Under the emulated calm water condition, 8-QAM-OFDM offers a data rate of 111.4 Mbit/s at the aligned position, while only 55 Mbit/s is achieved using NRZ-OOK. On the other hand, effective communication can still be maintained at a high data rate of 11 Mbit/s when the photodetector is off aligned laterally by 5 cm based on NRZ-OOK modulation, leading to a coverage of ~79 cm. By utilizing OFDM modulation scheme, a data rate of 30 Mbit/s can be achieved up to 2.5-cm misalignment, leading to a coverage of ~20 cm. Furthermore, in the presence of strong waves (15-mm wave height, causing a scintillation index of 0.667), 4-QAM-OFDM modulation showed a better resilience to channel instability than NRZ-OOK modulation. Our studies pave the way for the eventual realization of communication across a challenging water-air interface without the need for an interface relay, which is much sought-after for implementing a robust and large-coverage underwater-to-terrestrial internet-of-things.