When exercising in the rowing machine some people experiences the palm of their hands getting blisters. Over time the blisters will form into calluses and reduce the problem. If the problem is severe try to experiment with ultra thin palm coated gloves ( paid link ).
Remember to keep the handle clean after use to avoid skin problems. You could spray a mixture of 4 parts water and 1 part methylated spirits on the handle and wipe over with a paper towel to disinfect the surface.