Arborcoat stain, made by Ben Moore Paint, is a very good product and costs about $50/gal. On pressure treated lumber no primer is needed. Sometimes two coats do it a lot better for a deck that's in tough shape and the first coat is like a primer/1st coat.
Is good to initially spray the deck with a mildew/mold killer like JoMax mold killer, one day before staining to eliminate the old mold.
The reject shelf at Heath's Hardware in Center Harbor usually has a color rejected gallon of Arborcoat for the bargain price of just five dollars which can be a super steal-of-a-deal. ....

.... that came from some picky-wicky waterfront house in super-duper-rich Moultonborough! ....