Hi @Samnet,
Looking at the timber, I don't think it is necessarily at the point where it needs to be sanded back.
I would suggest you pick up some Cabot's Deck Clean, which contains oxalic acid, which is an effective timber stain remover. Mix it to a 1:4 ratio, following the instructions on the packaging, then scrub your deck with it using an Oates Long Handle Deck Scrub Brush. After scrubbing, allow the solution to sit on the deck for 10-20 minutes before cleaning it off with a pressure cleaner or garden hose with high water pressure.
After cleaning, reassess the deck. If the dark stains have been removed and things are more uniform, you should be fine to coat your deck with the Aquadeck. If the stains have not been removed, then you may need to sand it back.
Based on what I see, I'd be surprised if a good cleaning didn't remove them, but it is hard to say. I'd give this a try and let us know how you go. We can certainly assist further if required.
Jacob
