Going down to sea is symbolic of the descent of the soul to the physical world and its journey through life. For this world is compared to the stormy sea filled with "many waters." These mighty waves are referred to in Shir HaShirim:
Many waters cannot extinguish the fire of this love, nor rivers wash it away, (Shir HaShirim 8:7)
on which the Alter Rebbe lends amplification:
The "many waters" are the tribulations of earning a livelihood and the mundane thoughts in all worldly matters.... Despite all these problems they cannot extinguish the love...the hidden love which exists in every Jewish soul by its very nature and which radiates from its G-dly soul. (Torah Or, beg. Noach)
What course must one follow to be saved from these perilous waters? Go down to the sea in a ship! The good ship of Torah and mitzvos. On these ocean liners the Neshamah will chart a safe course through the stormy sea and reach its destination in peace.