Actually, there is something major missing from the replies you have been given, Fred, relating to how rowing machines (as opposed to rowing with oars) affect the abdominal muscles.
It is quite natural on a rowing machine to draw the pulley baton all the way into the stomach with some momentum at the finish of the stroke, with prior flexing of the abs in order to take the fairly light impact. This flexing can be stronger depending on how much momentum you want to add to the finish. The abs then relax again as you move back into the catch position.
This abs flex built into the rowing stroke is very effective in stimulating the abs and the whole core, and so I would disagree that rowing is not an excellent way of developing the abs. Indeed, for a whole body workout, this final addition to the technique covers one of the key aims of many exercisers, to stimulate the core and abs as part of a general aerobic workout.
Indeed, in an hour of moderate rowing, you will do the moderate ab flex as many 1800 times (3 strokes per calorie, total 600 cals). Now that's a serious abs workout by any standards.
The important thing is to not overdo the momentum or to be late in timing the flex, otherwise you will end up winding yourself and perhaps even bruising your abs.
But good technique throughout the rowing stroke is necessary in order to avoid injury and get the most from the exercise.