URL search using two words

I'm trying to do a basic URL search that uses two words: Used Boats. There's also a Used Motors section on this site, so if I only use the word Used, they show up, too. http://www.masselsmarine.com/marineBoats.php?category_keyword=Used

I can't use Boats on its own because there are several categories that include that word.

If I use the URL http://www.masselsmarine.com/marineBoats.php?category_keyword=Used%20Boats, I get a Record not found error message.

Any advice, please?