Lots of assumptions and over simplifications going on.
How deep is safe during lightning?
It is NOT SAFE during lightning.
If lightning is striking where you are then you are in danger.
This means that lightning within 5 to 7 miles of where you are too. The strike can go a long way.
If you are in the water and can see the flash you are in danger.
If you are on the surface and can hear the thunder you are in danger.
The only way to be SAFE is to be where the lightning is NOT.
Now for some more practical points.
In SALT water, the lightning is spread out quickly so it doesn't take much depth to make you much safer than on the surface.
In FRESH water the lightning takes odd and unpredictable paths and is very dangerous to great depth. In fresh water you are pretty much always safer OUT OF THE WATER.
There are lots more variables.
Where are you?
Florida has poor conductivity in the ground so everywhere is dangerous unless you have a big, strong building around you with lots of metal in the structure.
What lightning strikes?
They can be from a few thousand amps to more than 100,000 amps.
The really big ones are so powerful we really don't know what it takes to protect against them, except to be elsewhere.
In any case, the worst place to be during lightning is on the surface of the water. Next worst is likely underwater. Best is ashore in a stout building or in a metal ship.
NEVER assume that you can be safe with lightning around.
Remember that even though you survived the strike and feel fine you may have other problems later on from the lightning. See;
http://tigger.uic.edu/labs/lightninginjury/overview.htm