Dylan Thomas' poems are full of symbols and images and this often adds to their complexity and ambiguity. You should also keep in mind there is rarely only one coherent interpretation of a given poem, and The Ballad of the Long-Legged Bait is no exception. What I offer here is one interpretation; you should consider other possibilities as well.
The most obvious reading of the poem is that it's about the possibility of salvation via the subsumption of sensual desire. The voyage of the fisherman can be construed as the process of purification. Let's consider some of the imagery in order to see whether or not this interpretation can be supported.
First, we have the fisherman using a girl as bait:
For we saw him throw to the swift flood
A girl alive with his hooks through her lips;
We are also told that "Sin...had a woman's shape". Consider, also, stanza 4:
The sun shipwrecked west on a pearl
And the moon swam out of its hulk.
The sun, here, can be interpreted as a symbol of masculinity and the...
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