The metal antenna carries the electrical charge to the radio, which does not have a good conductor to the water. Your hand is on the radio, or on metal connected to the radio. Your feet are on a wet surface, which is in contact with metal which extends through the hull of the boat to the water. Your body may then become the best conductor for the electrical charge. A second example is a sailboat.
Lightning strikes the mast.
The electrical current follows the mast or wire rope to your hands, through your body to the wet surface, and then through the hull to the water. Or, while operating a motor boat, the lightning strikes you, passes through your body to the motor, and then to the water. The rod is struck by lightning. The electrical charge passes through the rod, your body, then to the boat to the water. In all four examples you could be seriously injured.