The seascape through which the speaker of "The Seafarer" travels is as dark and stormy as his thoughts. And that's kind of the point – the natural world acts as a kind of mirror of the speaker's state of mind, especially in the first part of the poem. Even with the arrival of a beautiful spring, though, the speaker doesn't feel any better. At this point, his gloomy mindset clouds his view of the natural world. Spring? That just means it's time for another tiring journey. The cuckoo's song? Its voice sounds "sad," since it marks the beginning of traveling season. So is it that our speaker's restless state of mind influences his view of the natural world? Or is it that the natural world makes him feel restless?
The weather the speaker describes in "The Seafarer" mirrors his state of mind.
The speaker's gloomy state of mind influences the way he describes and interprets the natural world through which he passes.