For scuba-diving, should one know swimming well?

Yes, you should know how to swim, and swim well. If you do not know how to swim good, you are at risk for panic and possible drowning or getting the bendz (benz?)

You should try a couple of swimming professional lessons before try scuba. I'm not a good swimmer too but, before I got married I did scuba on Figueira da Foz (a small town here in Portugal) and I had some lessons from a pro. He taught me how what I should do if I found some trouble down there. Try to find someone who told you some tricks like those... Have a nice dive, it's so wonderful!!!