The dihedral was meant to stabilize the plite and enable it to have only one tether.
Thank you for your comments but I have completely abandoned this design.
It think the whole idea - having a plane which flies independently over long stretches with electric propulsion, carries all the tether and basically never lands is bad.
Instead I am now thinking about a vtol multiplane which can actually land well on water and uses its own propulsion only for takeoff and landing. The whole landing on water bit is so that one can have an enormous rigid kite.