When We Crossed the Red Sea Did We Stop for Snacks?

Last night at dinner the Little Rebbetzin fishes a piece of kombu (kelp, a type of seaweed) out of her dinner:
L.R. : Daddy, what's the blessing on this?
Me : Um....shehakol?
L.R. : No, it's ha adamah!
Me : But it's seaweed.
L.R. : It grows on the ground.
Me : At the bottom of the ocean. I'll look up the correct blessing.

I pull out Halachos of Brochos, by R. Bodner and there is no entry for seaweed.

Me : Um...it doesn't list it here, I guess Jews don't usually eat seaweed.

Now the kombu was not the main ingredient or the primary reason for eating the stew, so it was covered by ha adamah for the other ingredients, but seaweed can be eaten by itself or as the dominant ingredient in a dish, so I should know the correct bracha.

The Little Rebbetzin uses the argument the seaweed is growing from the ground, albeit ground covered with a lot of salt water, and should therefore be ha adamah.

My reasoning is that seaweed is generally held to a rock by a primitive root called a holdfast, and is not growing in the soil of the seafloor. I view that seaweed is similar to mushrooms which take a shehakol because the sages do not consider them growing from the ground but instead deriving their sustenance from the air (this is actually not botanically correct, but is the ruling). Similarly the seaweed is deriving its sustenance from the seawater and not from the rock it is anchored to.

If anyone knows or can find out what the actual bracha is, and the reasoning behind it, please let us know so.