I know of no measurements of this.
Water is a fairly good conductor of electrical energy.
In the case of lightning, multiply the electrical current by thousands, and for the ocean multiply the volume by as much as millions. My guess is that the average lightning strike would electrify a few hundred feet worth of water from it's strike point sufficiently to electrocute someone. However, some current is probably for up to a mile before it is fully dissipated. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.