I'm a pretty decent swimmer. I put 50m (164ft) but I honestly don't really know how long that is. I have a hard time with distances... if you told me to walk (or drive) for 100 ft, I would have no idea how far that would be! I would have to ask for a landmark instead.
Either way, I feel like 50m sounds reasonable and like something I could do. I might fall into the "can swim several 50m lengths without stopping" if I can ever figure out how long that actually is.