Does diquat make its way through the water column or should it be sprayed all over the surface? I have been told both ways. i am trying to treat totally submerged plants.

If the submerged weeds are very dense, OR the water depth at application site is more than 2 feet deep then it should be applied subsurface., NOT sprayed over the surface.

So, the answer that you were told is correct, but the depth of the water and density of the submerged weeds has to be taken into account.