Nearly under the bridge over the "West Pass" of the slough that connects Skagit Bay and Port Susan is a grassy access point.
I have not actually launched from here, but at low tide, it looks like it would be fine. At very low tide, there is still plenty of water here, (but is muddy/dry a little ways up to the north.
My biggest concern would be leaving my vehicle parked there. There is nothing around there and would be easy for somebody to break into your car.