I suggest that you see if you can drain some fuel from the
bottom of the tank to see how much water you get before you hit diesel. The black stuff is likely to be an algae. After you remove all of the water, you can buy a cheap electric fuel pump and some clear plastic in-line fuel filters. Attach a filter, then pump from the fuel line back into the tank until the filter looks nasty, then change filters untill it looks better. Go to a major boat shop, and buy a biocide or algaecide to kill the lifeform, add it to the tank, along with an additive that is used for boats that burn diesel when they are put in extended storage. It should disperse all of the remaining moisture, and may even treat the contamination.
Burn up the diesel, and when the reserve light comes on, you should be able to remove the filter in the tank. Clean, or replace it, along with the primary and secondary fuel filters (if you didn't already) and refuel, adding some more aglaecide, along with some Redline Diesel Fuel Catalyst, 85+, or a diesel fuel conditioner from a boat store, or even Stanadyne additives from a local diesel injector and pump repair shop. I would be shocked if it didn't run a lot better.
For more information, do a search on
http://www.google.com for contaminated diesel, algaecide for diesel fuel, etc...
As for the glow plug light, mine didn't light until I replaced the defective plugs. It works fine now.