Swim efforts fall under the category of the Rate of Perceived Exertion (RPE), where 10/10 is flat-out, as fast as you can humanly go or as hard as you can humanly push yourself.

With that in mind, here are the percentages translated:

100% = 10/10
95% = 9-10/10

90% = 8-9/10
85%= 7-8/10
80% = 7/10
75% = 6-7/10
70%= 5/10
Easy Swim or Warm-Up = 3-4/10