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.