If it is for rowing seat, you might consider fitting a foot rest to stop you sliding about the seat.
If it is for a seat used while sailing, then you should sail the boat flat.......
Probably the most widely available material would be smooth treadmaster (not the diamond-type!). Alternatively painting on International Deck Paint is quite good at making a non-slip surface.
Fine sand (for bird cages) sprinkled on wet varnish works well for a wood surface. Brush off the loose after the varnish has dried, then varnish again to seal.
I wouldn't put non-slip material on a thwart as it is likely to chew through your pants in short order. If you must have something to avoid slipping when treading on them, go to a dinghy specialist chandlery and get self-adhesive strip which has a textured surface that isn't too abrasive. I use it on the gunwales for grip when trapezing and on steps on the big boat.
If it's for areas where you will only step, never sit, look at carborundum grit tape used for safety treads on stairs in public buildings.