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;
And all I ask is a windy day with the white clouds flying,
And the flung spray and the blown spume, and the sea-gulls crying.
The line "I must go down to the seas again" is repeated thrice in the poem. It says that the speaker spent most of his time travelling on the sea. He can hear a continuous call from the sea. The call is a kind of emotional bonding between the sea and the speaker. On hearing that, the narrator feels that it is unavoidable and he must return to the sea.
Wild and clear call from the sea

He also wishes that there would be strong winds making the white clouds fly. The strong winds would blow the clouds away and make his journey more comfortable.
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Strong winds blow the clouds
He wants to see the fine drops of water blown by the foamy waves and wants to listen to the cry of sea-gulls. (Cry of sea-gulls)
Foamy waves
Meanings of the difficult words:
DeniedTo refuse to admit or accept something
Flung Move or push something suddenly or violently
SpumeFroth or foam, especially that found on waves
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.
Cry of Sea-gulls: