When it comes down to floorboards, they do not cost that much for a sheet of ply but they would last a lot longer if protected well.
First tip, round all the edges well, paint or epoxy will bond better to that kind of edging than a sharp square edge.
If you don’t want to spend on coating the whole of the boards with epoxy, do the edges and holes for the turnbuckles which is where the water will get into the ply as its the end grain.
Cheap ply is made up of large wide veneers which suck up water into the ply and rot it so sealing these edges is important.
You will find that when using two pot poly paint it will melt the sugar into the paint, gloss paint just holds it and so has to be very dry and hard before you walk on it, else you may roll off the paint from the board face with your feet. When dry, cook in the house till use!
You could use two pot to start then go to the nonslip after. B@Q do some international paint, give that a look!
Is this the only paint your use on the boat, is it all GRP and what do you use on the centreboard and rudder? Could you use that on the boards.