Hardware store epoxies may not be waterproof. Look for boatbuilder/marine epoxies. System Three and MAS are good; West System probably is a more expensive option.
When I re-did areas of floor on my Argosy, I used System Three on the plywood edges and six inches on top of the floor perimeter. And then I treated the rest of the floor-top (eg, under the water tank) with RotDoctor
- a high-solvent penetrating epoxy (ventilate nothin' -- gotta do this with a lot
of outside air circulation!). Don't count on System Three, MAS or West to penetrate. Encapsulating wood really is a vain pursuit -- moisture will still penetrate somewhere and then nothing can evaporate (resulting in accelerated rot). I left the underside of my floor untreated.
Marine epoxies can be dealt with in enclosed spaces. Nothing more than a slight ammonia smell. But be very good on glove wearing and wash any skin contact with un-set resins. Use respiratory protection if you power finish or power sand any epoxy treated area.
Time spent looking at window gaskets, roof penetrations & skin lap seams will be more important than going overboard on sealing the floor. Gotta take care of the 'they all leak' problem.