Wow!! I am seriously impressed. I bet I could do that, but it would take a lot of dedicated training. Like training for a marathon.
I’m a lot more likely to do a long swim like this than a marathon, though. I’m a terrible runner.
ETA: So the first thread that caught my attention after this one is the one about orcas attacking boats in the Strait of Gilbraltar. 
I don’t mind being in the ocean while scuba diving where I can see underwater, but I’m always a little freaked out by swimming on the surface of the ocean for long periods of time where you have no idea what’s below you.