you could as a last result consider hydrogen peroxide. seems crazy watering it to save it from being over watered, but the hydrogen peroxide fills the soil with oxygen and supposedly works great. good luck - do not know the author but i found this somewhere and have seen other articles that confirm it - check this out::
Hydrogen Peroxide (H2O2)
How to use it.
H2O2 comes in several different strengths 3%, 5%, 8% and 35%, also sold as food grade Hydrogen Peroxide. The most economical is 35% which we recommend be diluted to three percent before using, as at this high concentration it can cause damage to skin and clothing.
This will also degrade any pesticides or herbicides that might be present as well as any other organic matter. Well water can be high in methane and organic sulfates, both of which H2O2 will remove. Many disease causing organisms and spores are killed by Oxygen, the free Oxygen H2O2 releases is extremely effective at this. H2O2 will help eliminate existing infections and will help prevent future ones. It is also useful for suppressing algae growth. The free Oxygen atom will destroy dead organic material (i.e, leaves roots) in the system preventing them from rotting and spreading diseases
How Hydrogen Peroxide prevents root rot/over watering.
When plants are watered with H2O2 it will break down and release Oxygen into the area around the roots. This helps stop the Oxygen from being depleted in the water filled air spaces until air can get back into them. High Oxygen levels at the roots will encourage rapid healthy root growth. In a Hydroponic system H2O2 will disperse through out the system and raise Oxygen levels as it breaks down. Strong white healthy roots with lots of fuzzy new growth will be visible. This fuzzy growth has massive surface area allowing for rapid absorption of the huge amounts of water and nutrients needed for rapid top growth. A healthy plant starts with a healthy root system.
Food grade H2O2 can be diluted to three percent by mixing it one part to eleven parts water (preferably distilled). The storage container should be opaque to prevent light from getting in and it must be able to hold some pressure.
What to do if you already have root rot.
In Dirt:
Use peroxide water with an anti-fungicide and a high Phosphate fertilizer (9-45-15, 10-52-10, 0-60-0) for root growth. Or any other product with rooting hormone dissolved in it is helpful in regrowing roots and is strongly recommended. Water heavily until liquid pours out the bottom of the pot. This sound like bad idea, but it flushes out stagnant dead water and replaces it with fresh highly oxygenated water. Don't let plants sit in trays full of water, the soil will absorb this water and stay too wet. Don't water again until the pot feels light and the top inch or two of the soil are dry.