Why would you even consider a low/med carb cutting option when you will be performing, what sounds like, an exhaustive aerobic conditioning program?
This is not the time to try and multi-task your fitness programs. If rowing is your thing, you need to do what will promote the desired results given your goals. This would be an increase in total cals. with an accompanying invcrese in total carbs. (I know nothing of your current nutrition so this is based on the average diet many of us would be using.)
It will be very important to get enough protein to aid in muscle recovery.
And oh by the way, we do not 'top off' with fats. Fats are critical to our bodies--especially men--in regulating T-production and getting quality cals.
If your diet isn't 30% fats (min) you are short changing yourself. In fact with the sounds of this routyine--I'd go out right now and get a couple of bottles of top-notch olive oil and a couple of containers of a quality fish oil. Your joints and recovery systems will thank you.