Although not common, it is possible for a diver to suffer a significant hearing loss in an ear from a diving accident. This is an individual decision for each diver to make, assisted by advice from your physician. Generally, if the original hearing loss is severe and loss of hearing in the remaining ear would make the person deaf, then my advice would be not to dive. This advice is particularly true if you tend to have difficulty equalizing regularly.
Most physicians are very conservative, even diving doctors. Severe hearing loss in both ears is a great handicap and should be avoided, if possible.