Marine grade Plywood is probably what would be recommended but you can use a standard grade 3/4 sheet of ply but if you do use standard you should treat it with several coats of a good marine grade sealing varnish or paint. Carpeting would be optionalable. A few coats of non skid gritted deck paint isn't a bad idea either. It doesn't look as good carpet but it's very durable and it will do a better job of keep your feet planted. A word of caution though the gritted deck is hard on fish so if you go this route be careful to keep your fish off the deck to protect them for a safe release. As far as added floatation goes you can buy styroboard that has been compressed and sealed at Home Depot they should it in a variation of widths. You need to need to cut it to the right size for under your deck. I've seen boats where people have used expandable foam and shot it under their deck but I wouldn't recommend it because the foam doesn't expand evenly with any consistency. Plus when it expands too much it tends to pust things appart being heavy in some areas light in others. Good luck and post some pics, when it's when it's done. Maybe some before after shots.

JerryG