How we cite our quotes:
[…] I have suffered grim sorrow at heart,
have known in the ship many worries [abodes of care],
the terrible tossing of the waves where the anxious night watch
often took me. […] (4-7)
He's quite the insomniac, our speaker, keeping an "anxious night watch." A "night watch" is usually something warriors take when they fear an enemy attack, so it seems the speaker might be worried about some ominous invading force that we don't know about.
[…] cares seethed
hot about my heart-- a hunger tears from within
the sea-weary soul. […](10-12)
The seething hotness of the speaker's cares is a stark contrast to the ice-cold environment he describes in the previous lines. The "hunger from within" is violent: it "tears" the sea-weary soul. This last line tells us that the enemy threat the speaker is keeping watch for may not be an outside force at all. Maybe it's just his own restlessness and dissatisfaction.
[…] Indeed, now they are troubled
the thoughts of my heart, that I myself should strive with
the high streams, the tossing of salt waves— (33-35)
His heart's thoughts are troubled, which makes it seem like his heart has a mind of its own. His heart also totally has power over him, causing him to actually want to go back on the ocean, even though it's such a nasty, cold, and violent place.