Poet, novelist, dramatist and journalist, John Masefield's literary career was rich and varied, and although his reputation waned in later years, he is again being recognized for his wide range, encompassing ballads, nature poetry and mythological narrative, and for his attempt to make poetry a popular art. John Masefield
There has been much debate over the first line and it is indeed "I must go down to the seas again, to the lonely sea and the sky;" "go down to the seas" was a poetic way of saying going sailing and did not refer to any specific body of water, similarly "the lonely sea" was simply talking about open water rather than a specific sea.
Masefield's use of the word "trick" indicates a period of duty on a specific task such as handling the wheel or lookout.