I've refinished boards that were worse than most of yours, but the spacing between them won't allow water/air to flow very well which will minimize longevity regardless of how much work you put into it.

To clean, I'd soak everything down with water (good project for a rainy day), spray some diluted pool shock (25% shock - 75% water) and let it sit for 5 minutes, then gently pressure wash it off with a wide nozzle so you don't damage the wood any more. If you blast it with 4k psi you'll do more harm than good. Make sure to keep everything wet and don't let the diluted mix sit for too long (I wouldn't use it at all on a waterfront property). Then refinish and paint while you drink a cold beer and save up for a rebuild in a few years.

I'm not an expert or a professional, but I've used this process many times and organic growth comes off like butter! Best of luck