I’ve got a few suggestions for possible reasons.
Some people find the rhythmic sound of waves very relaxing and lulling, as well as the feeling of rocking back and forth in a boat. The effect could be partly psychological.
Fishing trips traditionally involve a fair amount of drinking beer in the sun, which is a reliable way to feel tired pretty early in the evening. So if any day-drinking is going on, that would explain a lot of it.
Standing up (eta: and sitting up, for that matter) on a boat, especially a smaller fishing boat, requires you to work your leg and core muscles in a way which is very different from standing on land. Actively keeping yourself balanced on a rocking boat all day long is actually a fair amount of exercise, and you might not notice yourself doing it or getting fatigued from it.
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