Let us remember that this poem gives us a specific instance of restlessness that is based around the speaker's compulsion in this poem to get off the land and experience the adventure and excitement of life on the sea again. Clearly, the origin of this restlessness is many long years of experience, so much so that the sea has become the "stable" existence and the norm for the speaker, with the land becoming abnormal and somewhat monotonous. The poem speaks of the kind of fascination that the sea exerts on the speaker:
I must go down to the seas again, for the call of the running tide
Is a wild call and a clear call that may not be denied...
The intense imagery of the poem obviously captures the desire of the speaker and also speaks of his extensive experience as a sailor. Thus I would say that the poem points towards restlessness as coming from taking somebody away from something or some action that they have been doing for a very long time and are incredibly passionate about. I must admit, having had a while when I was not teaching, I felt similarly restless and eager to get back into the classroom.