Two thirds of the surface of our planet are covered by water and are still poorly instrumented, which has prevented the earth science community from addressing numerous key scientific questions. The potential to leverage the existing fiber optic seafloor telecom cables that criss-cross the oceans, by turning them into dense arrays of seismo-acoustic sensors, remains to be evaluated. Here, we report Distributed Acoustic Sensing (DAS) measurements on a 41.5 km-long telecom cable that is deployed offshore Toulon, France. Our observations demonstrate the capability to monitor with unprecedented details the ocean-solid earth interactions from the coast to the abyssal plain, and to record regional micro-earthquakes. The measurements consist in longitudinal strain-rate measurements along the cable with a spatial resolution of 20 meters and a temporal sampling of 2 kHz. These records allow us to observe the generation and progression of Scholte waves at the ocean-solid earth interface near the coast. This phenomenon is a major contributor to the primary microseismic peak. At depths greater than 1000 m, we measure Scholte waves with twice the frequency of the swell, which results in the secondary microseismic peak. These unprecedented observations pave the way for a better understanding of interactions with ocean gravity waves and complex bathymetry. Finally, DAS offers very high sensitivity to seismic waves (e. g. a micro-earthquake of magnitude 1.9 located 100 km away) whose signal characteristics are comparable to those of a coastal seismic station. This last result opens the way to rapid global coverage of oceans and coastal margins with seismic sensors.