Sushi, gym work is useful, but I would ensure to maintain some intensity in your training. If you have been doing interval training during the racing season I would continue with those sessions, but halve the number of intervals or duration. For example if you did 5 x 6mins on Tuesday and 2 x 20mins on Friday, then do 3 x 6mins (or 5 x 3mins) and do 2 x 10 mins. If you keep the intensity, but reduce the volume/time, that will ensure you hang onto most of your fitness in my experience. Any other rides at the weekend are helpful (e.g. riding with friends), and gym work can be used to build strength and improve overall body strength (back you and abs especially).
Sushi, as a rower myself I've often seen many people get on rowing machines and get no benefit because of poor technique. If you have any friends who are rowers get them to show you haow to do it properly. If you are using a Concept 2 rowing machine, you can do some great workouts which will really help your cycling.
You can mix up your "rowing" through the week just as you would with cycling. Some days hard, some intervals and some just easy. Set some goals for times over 2000 metres or how far you can go in a 20 or thirty minute session. Another method is to use the power metre (Watts) and work at a set power output while maintaining your heart rate in certain zones as you would on the bike.
Combine this training with some indoor cycling. By next summer you'll be jumping out of your skin.
Spider, I rowed seriouly a few years ago and can say it was good to get back on the old concept 2! I had to concentrate on bum-shoving to start but the stoke are quite clean (I'm suprised). Currently doing 20 minute sessions and managing about 4700km. This is some way adrift from the 5400 I used to get back in my hay-day... ;0(
I'm mixing it with 20 minute stepping sessions, hopefully this will give me the full range of motion I'm looking for.