Dishwashers are sealed closed when not in use.

It sealed around the edges and mechanically on rollers, rolled up when the pool was in use. 

They are only officially witnessing that you are the person signing the POA document, the they "seal" or rivet the document.  

Make sure you or your contractor seals off corners and door jams with foam since it spreads like a thick liquid and will find holes/cracks to drip down into lower levels. 

My property manager truly dislikes them since the day I told him they gave me a much more reasonable quote than the contractor he recommended me whom I did use on another project...Is it a bad thing that the some fence boards were used if they will be sealed and painted?

There are other options, like adding a small vent fan to vent the crawl space to the outdoors, putting a dehumidifier in the crawl space, and having the crawl space "sealed" - basically spreading out big sheets of plastic on the ground and running them up the inside walls.

How can I improve its support, seal it and prevent further erosion? 

Replace seals on your faucets so they do not leak3.

Last resort could be to dig out the sides and seal it on the outside that way but yeah, put a dehumidifier down there with someplace to drain the water it collects and call it a day.

I've forgotten what the best product would be to seal over the subfloor.