If I sold a used boat to a private person "as is", do I now have to by law pay for any repairs?

Assuming you did issue any warranty or guaranty (which it appear you did not), then you should only be responsible for repairs in the event of knowing misrepresenation. That is, say you said that the boat's motor was in good repair, but you knew that it was not; that misrepresentation could make you liable to repair the motor. So if you were aware of some problem or condition but did not disclose that problem or condition to the buyer, you could be liable for it; you would not be liable for conditions which you were unware of and which you should not reasonably have been aware of.