First, it's important to acknowledge that the speaker of the poem holds no idealized fantasies about life on the sea. He doesn't paint a picture of calmness and peace while out on the open water. Instead, he describes a cold and lonely journey of isolation:
He does not know this fact
who dwells most merrily on dry land—
how I, wretchedly sorrowful, lived a winter
on the ice-cold sea, upon the tracks of exile,
deprived of friendly kinsmen,
hung with rimy icicles.
He considers how his choice to journey on the "ice-cold" sea makes no sense to men who live on land. The journey is sorrowful and physically demanding. He returns to this idea of loneliness and of the divide that exists between him and those who live comfortably in the city:
Therefore he really doesn’t believe it—
he who owns the joys of life
and very little of the perilous paths, living in the cities,
proud and wine-flushed—how I must often
endure on the briny ways wearied.
Those who live civil lives, safe in the the comforts of society, look upon the speaker with confusion and even scorn. They cannot understand why he gives up a life that could be filled with people and comforts such as wine in order to journey again into the "briny" life on the sea.
Ultimately, the speaker feels that a comfortable life on land is dangerous, because it continually pulls him toward things that are not pleasing to God. Instead of seeking earthly comforts, he seeks the joy of the Lord:
Therefore they are hotter for me, the joys of the Lord,
than this dead life, loaned on land. How could I ever believe
that earthly weal will stand on its own eternally?
The speaker realizes that his discomfort is temporary and that the men who follow their earthly desires will ultimately fall and suffer an eternal condemnation. He is willing to place himself on the sea in order to separate himself from these temptations. He elaborates by noting that
Every man must keep himself with moderation,
to those beloved and those he deadly hates,
even though he may wish them be filled with flames
or burned up upon a pyre,
his own confirmed friend. Outcomes are stronger—
the Measurer mightier still—than the thoughts of any man.
This life of solitude on the sea keeps the speaker in a state of moderation. He believes that if he interacts with mankind too much, he will grow a "deadly hate" toward some of them, and that would lead to an outcome worse than anything mankind could imagine.
Thus, the speaker prefers the physical life of isolation and weariness on the sea so that his eternal soul is spared.