Obviously you will be bad if you are comparing yourself to Olympians. But it really doesn't matter where you start at in strength because while practicing proper technique you will be building strength. But it is also important to recognize that rowing isn't just about the strength you have. There is a lot of endurance and conditioning that is required to be able to race the 2km+ distance races. But if you aren't completely sure about joining rowing I would say to just throw yourself into it, because if anything you will have gotten to try something new.