Replacing floor in 1996 Sea Nymph

Hey guys, I purchased a 14 ft Sea Nymph and I'd like to replace the floor. However, doing so (to my knowledge) would require me to remove the front and rear bench, along with the two side compartments in the middle row. I've attached some pictures to give you an idea of what I'm working with. My concern is removing the rivets to get the bench's out and then having to reinstall them. These are not the typical blind rivets and I don't have the tools to replace them as they were. Do you guys have any suggestions as to how I should approach this issue? Is it possible to do this without removing the bench seats? Any help would be greatly appreciated!

What's the floor made out of?

Looks like you can detach seats from floor then remove it leaving seat fastenings to hull intact?

I am pretty sure it's marine plywood. Definitely the original flooring, but it is collapsing throughout the center of the hull right above the keel. Which basically tells me that there is no structural support under the middle of the floor. At first I thought that I could simply cut the floor in half length wise, and install it in two separate pieces, but without any structural support down the keel, it would definitely create a weak area. I could possibly put in some braces or supports, but that would really only be adding unnecessary weight, not to mention it could prevent water from draining adequately to the back of the boat. I think at this point I am just going to invest in a pneumatic hammer and take the bench's out and reinstall them. I was hoping to get away with blind rivets, but for the most part I've read that using them below the waterline was risky. Do you have any experience with replacing rivets below the waterline?