Go to where a bunch of 40 foot racers dock. Fish out their old lines from the garbage. Put in a mesh bag and wash a couple of times in a washing machine. Cut out the lengths you need and melt and whip the ends.
Alternatively, use climbing line that mountaineers use. The loads are going to less on a 20 footer than on a plummeting climber, and the outer covers are friction-resistant. A guy who runs a window-washing firm just donated hundreds of feet of 1/2" to our club because he has to "retire" it every two years by law. Good enough for my staysail sheets? Hell, yes it is, and for half a dozen other things aboard as well.
If it's a 20 foot slow boat, you won't see much benefit from new line, and "old line" for racers is frequently good enough for smaller, dinking-around boats.