Theory:

I must go down to the seas again, to the vagrant gypsy life,
To the gull’s way and the whale’s way where the wind’s like a whetted knife;
And all I ask is a merry yarn from a laughing fellow-rover,
And quiet sleep and a sweet dream when the long trick’s over.
Explanation:
 
The speaker wishes to lead a wandering gypsy life where no one can disturb him. He expresses his feeling that he wants to be free like a seagull and whale. Seagulls spent their lives over the shores.
  
seagulls.jpg
Seagulls over the shores
 
Whales live closer to the surface of the sea and wander it throughout their own life.
 
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Whales over surface
 
"Wind's like a whetted knife". This line signifies the winds cooling sensation makes him feel that it is much powerful as a sharp-pointed knife. The poet compares the wind to a sharp knife.
 
knife-1088529_1280.png
Wind is compared to a sharp knife

The speaker wants to hear exaggerated stories from a fellow wanderer. He likes to take a good sleep and have a sweet dream at the end of his journey. He wants to be free from the busy world.
 
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The speaker likes to take a sound sleep
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
VagrantA person without a settled home or regular work who wanders from place to place
2
Whetted Sharpen the blade of a tool or weapon
3
ExaggeratedTo make something seem larger, better than it really is
4
RoverA person who spends their time wandering
5
Trick A period of stay on the ship after the voyage
6
Yarn
A long or rambling story, especially one that is impossible
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.