Theory:

I must go down to the seas again, to the lonely sea and the sky,
And all I ask is a tall ship and a star to steer her by;
And the wheel’s kick and the wind’s song and the white sail’s shaking,
And a grey mist on the sea’s face, and a grey dawn breaking.
Explanation:
 
The speaker wants to travel desperately on a lonely sea without any one's company. He wishes to see only the sky over it.
 
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The speaker wants to travel on a lonely sea
 
The speaker expresses his feeling that he needs a sturdy ship to travel in the never-ending sea. He uses the phrase "a star to steer", which means that the sailor wishes the presence of star in the sky. It shows him the right direction that he needs to follow.
 
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Stars lead the right path
 
When huge tides occur, the speaker kicks the wheel, and he assumes that the wind sings a beautiful song. The movement of the ship makes him enjoy sailing on the sea.
 
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The speaker enjoys sailing on the sea

He intends to enjoy the early morning mist that rises from the sea along with grey dawn breaking on his adventure.
 
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Morning mist rises from the sea
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
DesperatelyUsed to emphasise the extreme degree of something
2
A star to steer The north star is the pole star which tells the sailor where North is and thus they can steer the ship correctly
3
SturdyStrong and healthy; that will not break easily
4
Never-endingSeems to have no end; endless
5
AdventureA thrilling experience
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-7. Sea Fever-John Masefield (pp. 93-96). Published by the Tamil Nadu Textbook and Educational Services Corporation.