This is a tough question to answer. First of all, lightning strikes on the ocean are rare. We also know that lightning strikes the highest point, which would be a boat on the water or a person standing on the shore. In general, the recommendation would be that if you are on shore, get under shelter if you can't lay down flat. If you are on a boat, get inside the cabin if you can, unplug and stay away from all electrical equipment, stay away from all metal objects, and crouch down in the center of the cabin. Wait until 30 minutes after the lightning is finished before diving.
If you are already in the water when the lightning begins and if you have an adequate air supply, then the safest place for you to be is under the water. When lightning strikes water most of it is distributed across the surface, so by staying under, you are more likely to avoid being electrocuted than if you were swimming on the surface.