I'm looking for the poem that I believe contains the line.."I must go down to the sea again. To the dark and lonely sea..." I cannot remember any of the rest. I thought it might be Mansfield, thoughunable to locate under such author.

Sea Fever
by John Masefield

I must go down to the sea 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.

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;
All I ask is a windy day with the white clouds flying,
And the flung spray and the blown spume, and the seagulls crying.

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 a quiet sleep and a sweet dream when the long trip's over.

Dear Julia H
You'll find that the piece you are looking for ("I must go down to the seas again, to the lonely sea and the sky...") is Sea-Fever, by John Masefield, contained in many anthologies, such as The Nation's Favourite Poems, published in 1996 by the BBC, and reprinted many times since.
Enjoy it when you locate it!