Trying to remove a Sulfur (Sulphur) smell form our potable water system on a boat. I have been told to use bleach but people are vague as to the mixing ratio. Any thoughts?

Not sure of the holding tank volume on your particular boat. I would use 1/2 cup per hundred gallons, if tank is smaller, go with 1/2 cup. Let sit 24 hours and then drain. It does not take much but you may have to repeat periodically.