Watermark is a freely available benchmark for physical-layer schemes for underwater acoustic communications. It allows researchers to test and compare algorithms for the physical layer under realistic and reproducible conditions. The benchmark is a shell around the validated FFI channel simulator Mime, which is driven by at-sea measurements of the time-varying impulse response. The first release of Watermark is issued with a library of channels measured in Norway (two sites), France, and Hawaii, offering three frequency bands (4-8, 10-18, and 32.5-37.5 kHz), single-hydrophone and array receivers, and play times varying from 33 seconds to 33 minutes.