Have you checked your local building code? Lots of costal areas specific requirements for height or allowing surge to go under the structure. Do you have flood insurance? Your actions will also affect that coverage.

Are you going to encase all of your plumbing in concrete? If so, you better replace it all first. You will likely make lots of your electrical cables unaccessible since most of it is probably run from underneath rather than from the attic. How do you feel about that?

OK, enough negativity. A slab under the house is far superior to an open space because you don't have all the air leaks to worry about, you lose a huge source of humidity transfer from humid air and you couple the bottom of the house to a moderate temperature mass instead of the highly variable and extreme temperature outside air.

Just one more question, was the floor insulated?

You couldn't pack in dirt there because it couldn't be sufficiently compacted and as previously mentioned, would probably violate storm surge requirements. You think it would be expensive to do piers, just wait until the whole thing falls down and has to be rebuilt! And frankly, with that much rot happening, it would probably be the best solution. The joists cannot be the only thing affected by the moisture issue. The subfloor and framing above it have to be affected as well.

Thanks for the good input!
To answer some of the concerns:

1) Yes it is covered by flood insurance, but because of it's age, the house is "grandfathered" when it comes to having to update to current code regulations. It was built nearly at ground level, long before there were breakaway walls or surge ports built into the lower walls. I'd only have to update to current codes if over 50% of the home's value was being spent on renovation.

2) The areas of the house affected are the two bedrooms and the living room. Thankfully there is no plumbing under these areas and the HVAC and electric is all fed from above.

3) If I were to cut out the floors and joists from above (leaving the load bearing walls along the perimeter of these rooms intact) could I just bring in fill dirt and then rent a vibratory compactor? Cover that with a sheet of polyethelene and then pour the concrete slab over top. (maybe add some rigid foam for insulation before pouring the slab?)

Yes, it would be best to lift the old girl up and address all the rot issues, but that's not financially possible right now.

@WetDogSoup May I know what you ended up doing ? My scenario is very similar to yours and I am planning to convert from wood floor joist to concrete with filled crawl space as well.

TY