Not a good idea to screw down floor boards - too slow, unsightly and a bugger (impossible) to remove once they have rusted in or lost their slots due to wear or heavy handling. Most zinc coated or other "corrosion resistant' screws will eventually rust in , or surface oxidise a bit, by virtue of moisture in wood. Lifting a board then means breaking it out and removing screws afterwards with a mole grip. I know this cos I've had to do it many times. A good basic rule of woodwork is to never use a screw where a nail will do. In this case round 'lost head' nails, skewed in at the board ends to meet the narrow joist.
cheers
Jacob