So I decided to build kind of a fishpond in my village and I also placed barrels there to make the fishers stay in the vicinity of the pond. Sadly, one thought it was a good idea to go swimming with the fishes. As visible in the attached screenshots (names contain time), he could not get out for 15 minutes, whilst he really looked like he tried to. Even though there are stairs as an edging, I had no problems jumping out. Eventually the villager made it as well, after a little pushing. Hence, the issue is not that villagers are unable to overcome this height.