I used 3/4" ACX. I think it cost about $40 at the local True Value for a 4'x8' sheet.
Made a template with cardboard first to dial everything in.
This is the only shot I could find of the running boards before I finished them. I ended up making the cutouts for the oar mounts a little wider for more flexibility with placement. My wife needs them a little closer to the seat when she rows. Routed every edge (top and bottom) with a roundover bit. Painted the bare wood with a few coats of Behr DeckOver.
I run 1' NRS straps through the holes to attach the running boards to the frame.
The DeckOver held up pretty well last season. Touched up a few worn spots this season. I like it because it has just enough texture to provide grip when walking on the boards. Amount of texture can be increased with rollers of different length naps and by varying the rolling technique.
The only thing I'll do differently this season is I might put a thin piece of closed-cell foam under any ammo cans I strap to the running boards. Last season the corners of the ammo cans wore through the paint.