If they were mine I would do the following. In a soft bottom inflatable the bottoms of the floor boards are pretty much always going to be wet. I would start the project by sealing the edges of the boards with epoxy. Epoxy coating the entire floorboard is a mixed blessing. If well done it will seal the wood, but if the varnish on the top ever breaks down the epoxy will fail quite quickly when exposed to the sun. Also, epoxy coating one side (the bottom) but not the other will likely cause the boards to warp due to differential water absorption. So other than the edges I think I would leave the epoxy in the can. I would paint the bottoms of the boards with three coats of something like Interlux Bilgekote. That paint is formulated for wet areas and will hold up well. If you want the tops of the boards varnished, then use a quality marine varnish. Two coats won't last. I would go with at least six coats. I would also build in some non-skid for the last two coats. Non-skid in varnish is easy. Tape around the perimeter of the area you want to be non-skid. Coat with varnish then sprinkle on some sugar. After the varnish drys rinse the surface well to dissolve the sugar then put on a cover coat of thinned varnish. The result is a non-skid varnish finish. If you don't like the result it is easy to sand it off and recoat with varnsih.

However, for dinghy floor boards I would probably paint the boards with a quality paint and put some non-skid in the last coat. When I built new floorboards for my ancient Quicksilver dinghy I painted them and they held up very well.