First post ever been lurking for a long about a year.  I'm a newbie to boats.  Me and a friend bought a 76 Sanger Super Jet a little less than a year ago.  Took the seat out to find a big rotten hole in the floor.  I was going to just patch the hole in the floor but after closer inspection found that the floor had been replaced before and was not done very well either.  I ended up removing the entire floor to find more damage from foam under it that held water like a sponge.  The floor that I removed was not even in the location of the original.  It was just set on top and glassed in really crappy.  Heres some pics of my progress. 
I plan on coating the bottom of the floor with a coat of resin before glassing it in.

awesome work!

To fasten the floor I added CAB-O-SIL from TAP Plastics to the Resin until it was a Peanut Butter Consistency.  I layed it down on the parts where the floor contacted the hull and put about 10 boxes of drywall mud on there overnight to hold it down.  Next I used a layer of 4 inch glass tape over all the seems then another layer of 8" tape over that.  Last I used a 50" sheet over the whole floor.  It is hard to tell in the pictures that it is there.  It just blends in.  It feels much more solid then ever before and it really was not that hard of a job. 

I brushed a coat of resin on the entire bottom and edges before putting it down.  I don't know if I would bother with the water sealer if you coat it with Resin.  Its probably not necessary.  My old floor was a rotten bad hack job that came out in a million pieces.  I used cardboard to make a template.  It really helped to make an exact fit.  to keep the gaps between he hull and the floor to a minimum.