What themes does Philip Larkin's poem "Seventy feet down" explore?

Philip Larkin’s poem beginning “Seventy feet down” is the
second poem in a triptych of works titled “Livings.” In this poem, the “living”
described seems to be that of a lighthouse keeper, who looks down on the sea far beneath
him. Immediately, then, one of the themes suggested by the poem seems to be isolation. 
The speaker is isolated because the lighthouse is probably in some isolated location
(perhaps even out on an island, surrounded by the sea). The speaker is also isolated
because of his extremely lofty location, not only far above the sea but also distant
from other forms of life, human or otherwise. Finally, the speaker is isolated as well
because he seems to be the only occupant of the lighthouse. No other occupant is
mentioned or even hinted at. Thus the poem seems to deal with a theme – alienation –
that is quite typical of Larkin’s work.

The speaker of the
poem is mainly a distant observer of energetic action taking place beneath him.  He is
not the initiator of, or participant in, any energetic action of his own.  When he
exclaims, “Running suds, rejoice!” (5), the exclamation only seems to emphasize, by
contrast, his own lack of movement and his own apparent lack of joy. The same pattern is
repeated with another exclamation at the end of the second stanza: “Creatures, I cherish
you!” (10). Earlier he had addressed the sea itself; now he addresses the creatures who
survive in the sea by clinging to rocks. Both exclamations, however, serve only to
emphasize the speaker’s distance from the things he
describes.

The atmosphere of the poem literally darkens, as
does its tone. The speaker is connected to the outside world only tangentially, through
technology:

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Radio rubs its
legs,

Telling me of elsewhere . . .
(14-15)

Even the news the
speaker hears from the radio is news connected to his job. It isn’t as if he listens to
chat shows or takes pleasure in broadcast music. His job defines his life, his very
existence, in ways that are truer for him even than for most people. At night, all he
can see is snow falling, like moths, as the beam of light from the lighthouse travels
around in the sky over waters that are now “Leather black” (25). Both the darkness and
the emphasis on winter help imply, again, the speaker’s lonely life. The speaker eats
alone and, as the poem ends, sees lit-up ocean liners moving ever farther away from him
– one last example of the theme of loneliness.