Strength training always seemed to help me. Back in highschool, I usually worked out around 3.5 hours of cardio ( stairs, runs, ergs, rowing ) and a hour after for strength training. It really depends on where you are in your season. If you are in sprint season, strength training isn't supposed to be that fantastic for you. Mainly, strength training is good over the summer, or fall ( unless you are a school/team that is highly involved in 5k/6k season ).
Some good exercises are deadlifts ( probably the best ), Front Squats, hanging leg lifts, any type of row, leg presses, pull ups, and some minor form of pec exercises. Just remember, the bigger the muscle, the more important. Also, make sure you don't ONLY work out rowing muscles, since this will harm your posture and musculature ( having overpowering back muscles can really hurt your pecs to where a simple move could make you pull a muscle ).
Also, some whey protein would be beneficial. After every workout, I would down a whey protein drink ( if I had time, or wasn't lazy, I am college student give me a break ). My favorite kind/flavor is http://supplementlocker.com/gold-standar-100-whey-chocolate-mint-2-lbs/
It tastes pretty damn good, and the brand seems to be pretty well reviewed. If you don't trust whey protein, a good dose of chocolate milk after workouts will help you pack on some muscle.
Also, if you are trying to gain weight, really start eating more food ( healthy food, please ). Trying to gain weight while rowing is like a oxymoron. Good luck, fellow rower!
College Rower - Yay