How do I clean a deck over a lake before staining?

If your pressure washer is splintering the wood you have the pressure too high. You could do a light pressure wash over the wood to loosen things up and then change to a stiff garage floor brush/broom and scrub with that. Two deck cleaners I could find were Eco-Safe and Safe Wash that might be less caustic. You might do some research online to find a deck cleaning product you feel is safe. There are choices.

Big-Box stores sell a product called Deck Cleaner.Check it out.

i am going to use 'oxiclean' to clean mine and then spray thoroughly. some would sand the whole surface. i will not with mine. just make sure all the excess is gone and surface looks flat. then use a deck stain product. if you hose down all areas around the deck first, a few times it will dilute the stuff that fills over. they do it this way to keep plants from dying. unless you live really close to the water, it should work.

I use oxygen bleach from Stain Solvers which is harmless to nearby vegetation and water. However, if you have a redwood deck, they recommend oxalic acid. You would have to research its effect on nearby vegetation. Check out the website here: https://www.stainsolver.com/cleaning-redwood-with-stain-solver/

Is this a deck that is close but not over the lake water or is it a dock that is actually over the water. The methods and cleaners would be different. Important to know.