This really depends on how deep the damage goes. I would first try sanding the area to see how far you have to go to get bamboo that is not water stained. You will then need to replace the finish layer. If the bamboo is stained it will be much harder to get the stain to match, but is the fnish is a clear urethane, or something similar, this should be pretty easy. Probably won't look perfect, but close enough. If all else fails, replace the boards.