Rowing at oxbridge of really good (where of=is for the benefit of normal people), and rugby players often at least try it out (some colleges have a "rugby boat" which is basically 8 rowers from their college rugby squad, and is usually the most technically crap boat although probably the most powerful!)
First term is a good introduction to rowing, with only a few outings a week (unfortunately they are v early in the morning usually so i hope you're the morning type) although because of the novicing popularity in first term, rivers tend to get crowded with people who just can't row and so your first term experiences arent always representative of what rowing has to offer. But then again, second term can be pretty miserable since it's so cold and wet and the hours of darkness are plenty which means seriously early mornings!
The best term of rowing is easily the last term, with lovely sunny evening outings, so stick it out until then if you can!
And the best build for rowing is a about having strong legs and back, it's a common mistake to presume rowing is in the arms...arms are weak in comparison to the legs and back although obviously you want a bit of muscle there! Rugby players do often have the potential for a good build, but will often have to shed a few pounds of fat (which happens anyway with intensive training).
Tbh at this stage I wouldnt think too much about build and stuff, first term is about enjoying it, and hopefully winning a novice regatta!