There are plenty of issues with their design from a surfing perspective but being on the surface or completely submerged wouldn't be one of them, if I apply my tiny bit of electrical knowledge. I'd say that's for covering their arses.
Take a simple circuit like this *, with a car battery hooked up to a 12V light globe and a bucket of water, probably with a couple of cups of salt in it.
Submerge the free lead from the globe into the bucket, submerge the free lead from the battery into the bucket. Globe glows.
Touch the lead from the globe to the surface of the water in the bucket, touch the lead from the battery to the surface of the water. Globe glows.
Leave the lead from the globe in the air. Leave the lead from the battery in the air. Globe doesn't glow.
The salt water is the conductor. The resistance difference between submerged and surface should be negligible, provided the circuit contact area is enough (and I would count a full trailing probe as "enough"). Air is not a conductor at these currents and voltages.
* Don't do this. Theoretically this should work but I haven't tried.