Can you lay marine grade plywood over a soft floor on a fiberglass boat and seal it and paint it and have a safe usable boat?

Yes, but it"s like putting a bandaid over cancer. You are not correcting the problem just covering it up. Best way is to cut the floor out and replace it, that a way you can see the stringers and see if they are rotten. Also to see if the foam is water logged.

So I guess even if I had to replace the foam and stringers I could still put down the plywood floor?

caseguru,

Plywood is about the only material that is really suited for floors.

However, paint will not completely seal it. You will really need to use fiberglass and resin. Coat the underside of the wood with resin completely, before you drop the floor down. Then cover the top surface of the floor with two layers of fiberglass.

Then you can seal it with gelcote or paint.

OH, BTW, don't use the green treated wood or the cheapest wood. USE Marine grade, or at the least, use AC grade wood.

Thanks Michael
I have some sheets of 3/4 marine grade that cost like $80.00 a sheet just stored in my barn and was wanting to put them to use. A guy gave me this boat and everything about it appears to be in good shape except the soft floor. Just what I needed another project.