But if they've been here more than a few years the humidity will drive them back here or to AZ.Sorry I went on and on, just new thoughts came to me as I hovered over "post".

Summers can be warm and humid but typically in the mid 80's to low 90's. 

A tenant is not going to do a good job if it’s in the 90’s and the humidity is just as high

As for addressing the issue, if the toilet is functional, I wouldn't replace it as mold would just likely regrow there if it already is and you live in a humid environment.

The only thing it requires is access to a small window to vent out the humidity.

I would not do this in a hot humid climate where it would be expected the windows would be open.

You are in the humid category so the vapor barrier should be on the outside (exterior) of the of the insulation. 

Also, any recommendations on what material to use down there - it is right on the beach and gets super humid.  

Look for the water source.In an attic, likely suspects are roof leaking amd humidity. 

I have 2 high capacity dehumidifiers that have been running at 50% for the past 2 years so the humidity levels stay low.The awful smell in gone but now I have a lingering smell that will not go away.It now has a faint smell best described as garbage mixed with perhaps used week old coffee grounds.