Just to add to Goran's advice...
Weight is another issue. Marine ply is half the weight of pressure treated. If you don't want to spend marine ply money; you can use regular plywood, seal it as well as you can and be prepared to do the job again.
When refurbing my boat, I used 3/16" aluminium for the floors. I'll never have to do it again but it's heavy too. And you think marine ply is expensive... LOL
Spend a little more (double) for the marine plywood, you'll be glad you did.