Epicenter of earthquake at sea - is it a point on ocean surface or sea bed?
Technically the depth is distance below the ocean floor, but bear in mind that unless there happens to be a cluster of seismometers close to the earthquake (unlikely in the ocean environment), the accuracy of depth estimates is only approximate. If you look at the table of earthquake depths in, for example, the Tonga Trench, you will see a few really deep earthquakes and many times more 'shallow earthquakes' at a nominal depth of 10 km - meaning 'shallow-ish'.