The definition of "good" is obviously subjective, but you've set a criteria of a 20 min 5k and seem to be looking for the equivalent swim performance. The men's World Record for the 5k (on a track) is 12:37. A 20 minute 5k is 158.5% of that. Men's World record for the Long Course (50m pool) 1500 is 14:32. 158.5% of that would be ~23:02. That converts to a ~22:38 1500 for Short Course Meters (25m pool) or a ~22:35 for 1650 Short Course Yards.
"They're made of latex, not nitroglycerin"
"They're made of latex, not nitroglycerin"