I have a wooden walkway that leads from my deck down to my pool. Some of the boards are rotten and I want to replace them. Is this a project for a Nail Gun (which is how everything is currently put down) or a Screw Gun? Reason I ask, is that while the current boards are put down with nails, I have to hammer in the nails probably once every 6 months in random spots? Would braiding on screws hold the wood down better and not have to be "re-screwed" every 6 months?
Decks have been built with screws for the last 30 years where I live. Nails are unheard of for the reason you mentioned. Your local hardware store will have a selection of epoxy-coated or galvanized screws in various colors that will serve you well. 2-1/2" is typical for 5/4 decking, and 3" for 2-by decking.
You can use a standard drill to drive deck screws. You don't need a screw gun, though the auto-depth feature they offer can save time on large projects.
If you want to use wood I would use pressure treated boards ( either 2x4 or 5/4 radius edge) and secure them with ACQ screws. They are either Brown or green depending on where you live. First thing to check however is the condition of the stringers (boards running perpendicular to the deck boards) to see if they have rotted out. If they are rotten, you should replace them or the nails and screws will never hold.
If you have a bigger budget, consider using one of the composite deck boards such as Trex. You won't have to stain them ever and they don't rot.
Wood screws are definitely the way to go. They will pull the deck board tight to the joist. I'm not sure what type of wood you have for your project, but this Fastenmaster screw would work for pressure treated, softwoods and redwoods.
If you do have a larger budget, a composite or pvc deck board would be a good solution to the rotting issue.