I use a plywood floor on my boat, with occasional maintenance it is still going strong after 6 seasons. Some of the early versions didn't last beyond 1 trip. Some thoughts on my experiences.
1. I considered marine plastic but was unable to afford it. High quality plywood at $60 sheet vs plastic at over $300. I can easily find plywood at local stores, the plastic required special orders, minimum quantities and extra shipping.
2. Exterior house paint is a much better waterproofing than spar varnish,
and a whole lot cheaper.
3. Waterproofing plywood depends mostly on sealing the edges and voids, which can wick water deep into the board. You can fill edge voids with 2 part epoxy wood putty or auto body putty. Then seal the whole board on all sides with a coat of epoxy thinned with denatured alcohol. (check out the wooden boat builder web sights for a 'how to') The epoxy doesn't hold up under UV, so you must topcoat with spar or paint.
4. Two 5' sections are a lot easier to carry, rig and transport than one 10' section. I found three bays worked best for my boat, one was custom fit for a dry box, one for a cooler, and the third for the passengers and rower.
5. It doesn't need to be marine grade plywood, as long as the glue is rated for exterior use. It's more important that it is void and knot free. Get the best quality plys (ie: 13 ply over 7 ply over 5 ply over 3 ply etc.) you can afford.
6. Pressure treated wood will not work well. Treated wood is rot resistance NOT water resistant. The treating process actual makes the wood more likely to soak up water, plus any splinters are toxic.
7. Home made wood floors and decks are adaptable. I have made many tweaks over the years. You can make repairs or changes with materials found at any hardware store.
8. You can buy high quality plastic in smaller quantities through commercial kitchen suppliers. They sell it for use as cutting boards and counter tops.