replacing floor in 18ft boat side console

If you can't find treated 5/8", just use a high quality exterior plywood to start. Once you cut it to shape and it's dry fit, then seal it in fiberglass resin. This will outlast the aluminum hull I assure you.

You can get a gallon of resin and hardener at most big box hardware stores.

You can save ordering time this way plus overall cost. Also you'd have to cut a "marine grade" board anyway. This exposes new material, making areas to invite rotting. Then you'd have to seal those areas.

I updated my website, so there's a bunch of pics of my past work.

http://www.anchormarineservice.com/photos.html

You can use a 2 part liquid foam to adhere the wood to the ribs. It's about $60 per cubic foot, but ideal for floatation and adhesion.

The spray paint circles and line were to indicate where it was left vacant of foam because of the odd way the ribs were laid out.

Otherwise that's all foam underneath. You couldn't remove that board if you had to.

I do a fair amount of floors every year. As long as you have some above average carpentry skills, you'll be good.

The wood thickness depends on what was in there initially.Use the same that came out. You can run into weight or strength problems if you change the thickness. This link will get you to my Facebook page. I have just started putting pics up of my past work. Further questions, feel free.

https://www.facebook.com/pages/Anchor-Marine-Repair-LLC/100928749988655