In Dylan Thomas' "The Ballad of the Long-Legged Bait," what is he talking about?

The poem is an allegory on death and salvation told in the story of a fishing boat.  The boat leaves shore and makes its way through a storm.  There are many mythological and religious references sprinkled through the poem, along with great visual imagery of the ship being tossed about on the waves as the storm becomes worse.

Don't assume that Thomas inserts any words simply for how they sound.  The sound of the language is important, but Thomas worked and re-worked his poems so that every word has a purpose.

Lucifer is a religious reference.  According to the Biblical book of Isaiah, Lucifer was one of three archangels in Heaven who fell out of grace with God when he attempted to take authority from God.  Lucifer already had beauty, wisdom, strength, power -- but he wanted more and wanted everything God had.  In his efforts, he fell out of favor and was cast out of Heaven.  Thomas makes the reference here to suggest the loss of qualities that we consider favorable.  The stanza continues to say Lucifer is dropping "out of the sides of the north."  The "sides of the north" is another Biblical reference from Isaiah that suggests God's holy temple (Isaiah Ch. 14).  In that Chapter, there are many references to the "great depths" below, which match the symbolism that Thomas uses in his poem.  The depths of the ocean beneath the fishing boat are analogous to the great depths of Hell to which Lucifer has fallen.  When Lucifer "drops out of the sides of the north," he has fallen away from God's favor and is cast into Hell. 

Venus is a mythological reference.  She was the Roman god of love, and the mother of cupid.  According to mythology, she was playing with Cupid's arrows one day and accidentally wounded herself with one.  Cupid's arrows had the power to make a person fall in love, so the line "Venus lies star-struck in her wound" suggests the overpowering strength of love.  The "sensual ruins" -- or anything "sensual" -- refer to our human body, which is weak in comparison to the power of death, of love, of the vast ocean. 

The final line of this passage, "White springs in the dark," suggests the lightning of the storm, but could also be a reference to light showing the way through darkness.  Hope.  Vision.  Redemption and resurrection.  In the midst of all this trouble, turmoil, the depths of Hell and the depths of the ocean, there is hope as "White springs in the dark."