One of the factors of ego depletion is decision fatigue. If the fisherman does not fish that day, he and his offspring go hungry, and if that decision is repeated they will starve to death. So not really a decision, not one that a rational being would make. If the web developer goofs off on reddit today, instead of looking for new contracting work he won't go hungy.. He can probably coast for a little bit.. Actually, he could potentially just go on welfare, but then he'd have to live in a worse part of town. If he moved in with friends he could work less hours a week and have more leisure time, etc. Lots of choices to make.
Novelty is another ego depleter. The fisherman has done it day-in-day-out for years, probably his father and grandfather as well. It's muscle memory and he's on autopilot. The web developer is constantly novelty fatigued, learning new clients' environments and the latest web framework fads which change month by month. His day has no routine.
As a contractor it is much more difficult for me, and probably I'm the most lazy and restless after coming off a project, with a decent balance in the bank. I am much more focused and mentally sure when the bank balance is low and I'm chasing down business because if I don't find something it'll get uncomfortable quick.
Contrast that to when I was in the gulf of Papagayo, miles from shore, on a small sailboat in the middle of a storm. There was no ego depletion. I never thought 'well, should I continue battling the sea to keep this boat afloat and going to my destination?' There's no ego there, only survival.
Does the Vietnamese fisherman have the harder life by most metrics? Absolutely. However there are downsides to the modern western world.
I don't think cold water is actually bad for digestion.
Not only that, but think of all the cultures/jobs where it is normal for people to work much more than the typical 40 hour work week we are accustomed to. Do 18 year old Vietnamese fishermen working 80 hour weeks suffer from "ego depletion" the same way 18 year old college students do in the US? Probably not. It may be an element of our culture / upbringing that we put more emphasis on leisure, and not some psychological or neurological "need".