How am I going to get this plywood out from under these side panels.

Take things apart.

I've always had to drill out those rivets on the side of the hull and remove the interior boxes and panels in order to get the floor out. Pain in the rear end but it's really the only way to do it right as far as I know.

Here's a few helpful hints though:
- Center punch the rivets and make sure you drill them out centered. If you don't get the drill bit centered then you'll be left with an oblong hole that will leak when you put in the new rivet. But...all of the rivets you need to take out should be above water line.
- Use a brand new drill bit that is the right size.
- I got an inexpensive but very effective riveting tool through harbor freight. It's like an air hammer and mushes the new rivets in place.
- Get a buddy to help you hold things in place when you're putting it back together.
- Practice on some junk sheet metal before you get started to get the feel of it.
- Make sure everything is lined up good before you get after it. It's hard to adjust things with snug rivets.

Good luck with it. You can make an old boat brand new again without too much difficulty.

I did a alumacraft I used to have. I just used a saws all and cut down the edge of the sides without cutting into the cross members. Just left what's under the sides in there. Then put trex down and calked between trex boards, then calked down the edges. Easy to clean and was not slick. Probably added weight to boat but actually turned out nice.

Zook, I have that exact same boat. I've never really paid any attention to how the floor is laid out as it still seems solid but I'll check it out tomorrow and see what it looks like. Man , I hate to think you'd have to take all the rivets out but it may be the case.

Remove the side panels. Replace the floor. Re-install the side panels.

Sounds easy but I've never taken out rivets on a boat.

my dad had an aluminum boat with floor and raised fishing deck , he had some pieces of the fishing deck got soft , he fitted good plywood drilled any holes then remover that and sealed it all up with fiberglass resin then carpeted it and put it in , it came out really nice and was still very solid a decade later when he sold the boat

it maybe wouldn't have to be resin although it makes it extra stiff , but seal the heck out of the plywood

If I was doing your boat I think I would cut the plywood , and seal it well then use textured deck paint and paint it , I don't like carpet

You can either grind off the top, then punch them out with a punch that will fit the hole diameter...or as was already stated, drill out the hollow half, then punch it out the rest of the way.

I haven't searched for it, but I bet youtube would have some good videos. It really is easy and fast. Practice on one that is out of the way, and go from there. Good luck.

Sent you a pm Zook, not sure it's much help though. Lots of rivets and I suppose to do it right, taking them out would be the best way. I've never done it but as others have said it looks like getting them out straight is the key.
Good luck.