Actually, I don't see why you can't use thermalite blocks above ground. Lateral water pressure decreases the nearer you get to the top so it rather depends how much of the pond is above ground. If it were me, I'd be inclined to beef up the thickness of the GRP and take it up over the top of the blocks to form a wide flange. This should proved plenty of lateral stiffness so that you're really only using the blocks as a former and something to fix your external cladding to.
I'd be more worried about the effect of polyester resin on insulation foam if it is expanded polystyrene or styrofoam.