1. Determination
In order to get in the boat or on the erg day after day, you must be determined and focused on what you need to do to get the job done. Determination is key because you are going to have to go through the workouts day after day. It can become monotonous after a while but you need to find a way to mix it up a little and get excited to finish the workout.
Determination is something that you are born with I think. Some people naturally want to win and they are determined to do whatever it takes.
2. Mental Toughness
You have to be mentally tough to be able to be an athlete. Really any sport requires this because you are pushing your body when your brain is telling you to slow down. It takes a lot to push past physical pain and to keep going until you are done.
I think it is possible for people to develop mental toughness even though to some people, it probably comes more naturally. By finding what you love to do and competing to do your best in it, you can find that you are more willing to go the distance to succeed.
3. Experience
Boat experience and race experience can make the difference for some coaches in what boat you will be in. If you have rowed a variety of boats in a variety of races on a variety of bodies of water, you are golden! I have found that having small boat experience, rowing in basically any boat there is (including the octuple [8+ rigged for sculling], the single, pairs, quads) will help to get you farther in rowing. It opens doors for you as a rower which in turn open other doors that can propel your rowing career!
Obviously you cannot just wake up with rowing experience. You have to be open to all of your options and be willing to learn something new for example if you row eights all the time, try rowing a single. Experience is something that not everybody can just have and is valuable if you are a high school rower trying to be recruited to a college.
4. Strength
Clearly, rowing is a strenuous sport that requires a good aerobic base as well as stamina and strength. You need to have strong legs to be a rower! As well as the legs you need good core strength as well as back and arm strength. Unless you are a rower, you might assume that it is all in the arms, but this, you will find is false. Your legs are the powerhouse that propel the boat. Just look at the quads of any Olympic rower and you will see.
Strength can obviously be built by lifting weights and gaining fitness. Anybody can build strength. By joining rowing as a novice and practicing for a year you will begin to build the strength that you need to be a rower and by adding a lifting schedule that is geared towards rowing you can continue to build up your muscles.
5. Motivation
Self motivation is another thing that most athletes must have. When there is no one around to push you who is going to? Well the answer is you. It's your job to get yourself on the erg in the winter. And it is your job to be able to pull out those hard all out pieces even if you don't have a coxswain to keep you going.
Motivation as well as mental toughness is something you've just got to have. You can develop the motivation if you have something you are working for, which most rowers do. Always looking for the few seconds to PR by and wanting to place and beat the competition are usually motivators enough. But you gotta have your teammates to rely on when your battling it out with another crew in the last 300 meters of the race. You need to know that they are not giving up and they are motivated by you and everyone else to make it across the finish line.