Oceanic crust is typically 6-10 km thick.
Continental crust is typically 35-40 km thick, but can be a lot thicker below mountains. High mountains also have deep roots that enable them to "float" higher on the Asthenosphere. The crust there can be up to about 70 km thick or more.
Note that not all crust covered by the sea is oceanic crust. Some continental shelves extend beyond the part of the continent that is above water. For example between Australia and Papua New Guinea and between Malaysia / Cambodia / Indonesia. The satellite view of Maps.google.com clearly shows how the continental shelf is extended beyond the landmass.