This is a poem that is all about the pull of the sea upon us and how the speaker of the poem experiences "sea fever" as he feels an uncontrollable desire to go to the "lonely sea and the sky." The "call" that he feels is obviously related to this and is mentioned in the second stanza:

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...

Having obviously spent so much of his life by the sea or on the sea, this die-hard sailor finds that he cannot live without it and that the "call" creates an impulsive desire to return to the sea. Let us consider how this call is characterised: it is a "wild" and "clear" call that cannot be "denied" by the speaker. Note how the words "wild" and "clear" create a picture of the wildness and emptiness of the sea, creating a spirit of adventure which is very attractive and characterises the speaker's thirst for a return to that adventure and wildness that he has experienced in the sea before.