With all respect, you are horribly mistaken with your assumption that mastering the rowing stroke is an easy thing, and considerably easier than the other examples you provide. The rowing stroke is a thing of beauty when done perfectly, but perfection is rarely achieved. It is something that you practice and train for everyday in hopes of eventually achieving. Having played all the sports you mention as a youth and having now been exposed to rowing I can say that all have a factor of ease in picking up and doing and doing fairly well in. Rowing a bit more so, simply because its very different than the other sports I had previously played and also because as long as you aren't in a single, which I'm not, you also have to depend heavily on your teammates. Rowing requires an incredible amount of hand-eye coordination, focus, intensity, and control. Puck-handling I'm sure is an extremely difficult process to become great at. However, there are thousands (maybe millions) of people who can do it well enough to play a game of hockey. Of course to do amazing feats requires a step above, but so does winning an IRA or an olympic game in rowing.
As for quarterbacks. I always found quarterbacks to be dumb and unsuccessful pitchers..... but that's my experience
I like to think about putting a player in another player's shoes. I think it tells a lot. Put a football player in a baseball players shoes. Do you think he'd fair alright? Maybe. Put a basketball player in a baseball player's shoes. I think he'd look like a fool (cough michael jordan). Put a baseball player in football....I think most would fare decently. Hockey players.....well, to be honest, I cant see them playing much else successfully.