One's body, which for this is equivalent to a sack of dirty water with lots of salt and such in it, would probably pass most of the leakage current from the heater. Current would flow from the heater thru many paths in the water (which is likely to be somewhat dirty) to your body, and then thru your body to other areas of the body and exit thru more multiple paths in the water to a grounded faucet and drain hardware. And the porcelin coating on a metal tub is likely to have many small cracks, creating more current paths....
