Sure, the poem is dedicated to Lowell's cousin who drowned at sea, but does that mean the poem is supposed to literally be from Lowell's perspective? Not necessarily—at the beginning of the poem, we find out that the speaker is part of the North Atlantic Fleet, which is part of the US Navy. Lowell, himself, was an objector to war, and never served. But the speaker is similar to Lowell in some ways; it's likely that the speaker is male, because Navy sailors at the time weren't usually women. Also, he's familiar with Nantucket and the whaling industry, as was Lowell, who spent several summers in the area.
In the seven sections of the poem, the speaker spends the most time in the Quaker graveyard, where he ponders the dead sailor that his fleet weighted down, and also all those lost at sea during the era of whaling. The poem takes place long after whaling has ended, and it could be set when it was written, in the mid-1940s, just after the end of WWII. He has grief over the sailor's death, but also expresses the belief that to challenge the sea—and by doing that, challenge God—is foolish and ultimately deadly. He's concerned that man's pride and corruption will spread over the earth, and warns that these sailors met their end because of this type of pride.