I tried this one on a few different maps until I realized that you can't have any kind of cliff face between you and the water.
For reference, I did this on my personal map, available in World of Wardens called "ta escape", but you can do this in any map where water is available to inmates or edit one of your maps to include water.
- First, you will need skill points in Swimmer to successfully wade into the water. To get the Swimmer category to unlock, it seems that you will need at least one skill point in Fighter and Athlete.
- It seems pretty pointless to have only a few skill points in Swimmer, as you will just swim for a second or two, drown, and wake up in the Infirmary. I would wait until you have a pretty significant amount of points in Swimmer before attempting this.
- Once you have enough skill points, simply walk into the water. A blue bar will immediately start draining indicating how much time you have left to swim. Keep swimming around you've gotten the achievement or head back to land to refill the meter. When I attempted this with Swimmer maxed out, I got the achievement before the bar hit 25%.
If you're using my custom map, there's a body of water you can use in the bottom-left of the map. In the bottom-middle is a Classroom with a Shop in it you can use to get skill points quickly. Make sure to recruit an inmate early and order them to 'Extort Convicts' to make enough money to keep your vitals up at the Shop.
Let me know if you have any questions or want to see any changes to this solution.