Decks, Patios, Porches and Docks - Some questions on protecting decking

I purchased a home a year ago and we've decided its time to repair a 300 sqft low deck (acts more like a patio) because the decking is rotting. After pulling up the existing decking, it looks like the framing is in great shape, so my plan is to just put new deck boards on. I'm leaning to Southern Yellow Pine due to budget concerns, but may go with a higher end wood.

Questions:
- Is it a good idea to stain the deck boards before installing them? I will not be able to get access to the underside after installation. Do I even need to coat the underside?

- Is there a good brand sealant/stain that anyone can recommend? We would want to see the wood grain through the stain, so I'm guessing a transparent stain would be best.

- Do I need to stain AND waterproof? I suppose this depends on brand/type of coating.

- What kind of gap should I leave between the deck boards to ensure proper drainage?

Thanks in advance for any guidance that you might have...

Some might consider it overkill to prestain all six sides of the board, but I'm kinda prone to overkill myself. Generally, the bottom doesn't get sealed, since most people do it from the top after installation. I see Cabot, Sikkens and Benjamin Moore all get recommendations here often. Deck stain is wood sealer/water repellant. Normal gap is often the thickness of the nails/screws you're using to attach the boards.

I you are using recently treated PT pine, you don't need to leave a gap between boards - it will shrink as it dries creating the gap. It may also complicate staining the wood but it might be worth the extra effort. What type of wood is currently on your deck?

The wood grain should always be pointed down - so make sure you stain the right side. Unless the wood is dry, I wouldnt stain the top side for a few months.