What did Mr Peggotty decide to do after he read Emily's letter?

From the text:

‘I’m a going to seek my niece. I’m a going to seek my Em’ly. I’m a going, first, to stave in that theer boat, and sink it where I would have drownded him, as I’m a living soul, if I had had one thought of what was in him! As he sat afore me,’ he said, wildly, holding out his clenched right hand, ‘as he sat afore me, face to face, strike me down dead, but I’d have drownded him, and thought it right!—I’m a going to seek my niece.’

‘Where?’ cried Ham, interposing himself before the door.

‘Anywhere! I’m a going to seek my niece through the wureld. I’m a going to find my poor niece in her shame, and bring her back. No one stop me! I tell you I’m a going to seek my niece!’