Clear Sealer for Garapa Deck

Apologies if the answer is already out there, but I couldn't find it.

I live in the northeast US and have a 4 year old, non-stained, grey, garapa deck. It is on the water (a bay, so not full salt) and gets full sun. It has slowly been developing black stains that some feel are mildew, while others say it is a type of discoloration not unheard of in garapa. A mild solution of bleach (~10%) and water lightens it, but does not fully get rid of it.

My question is, after I do get rid of the stains, I'd like to clear seal the deck, with something that will help prevent further staining of all kinds. I want clear, because we like the grey. It seems to be hard to find a well-regarded sealer for hardwoods that comes in clear. Any personal experiences and opinions are welcome.

Thanks.