How hard is it to replace the wood under the floor of a power boat that is getting soft spots from rot?

Not to hard, you need to cut the carpet in an inconspicuous place and peel it back exposing the soft or damaged area then cut out with a skill saw or sawzaw. Then replace with green treated plywood usually 5/8 or 3/4. Boat shops sell floor repair kits that include a special metal insert to support the new plywood and will also supply a fibreglass repair kit to put over where you made your cut in the floor. Once this is completed then you can re glue the carpet down. If done correct the repair will be as good as new and be undetectable

you will need to remove the fiberglass to replace the wood in refiberglass when done.or as idid cut all put whethertreted wood ond screw it down dont screw to far therw the boat

There is no way to answer this properly without seeing it. I my experience, it is never as simple as the owner thinks it is. Once you start the demo, I can almost guarantee you that you will continue to find all sorts of 'disturbing' things that need repair. The above suggestions are adequate, depending on the standards that you want to achieve. If you have never done boat work before, be warned, this can break your bank account, and destroy your ego. If you are a utilitarian boater, what the heck, give it a shot.