Green’s Dictionary of Slang

originally transported (e.g. to Australia); latterly sentenced to a long term in prison.

1859		Matsell Vocabulum 13: boated. Transported; gone to sea.
1873		Sl. Dict. 89: Boat, originally to transport; the term is now applied to penal servitude. To ‘get the boat,’ or to ‘be boated,’ is to be sentenced to a long term of imprisonment equivalent to transportation under the old system.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		(ref. to mid-19C) Mirror of Life 16 Jan. 3/1: In the old days, when a man was transported he was ‘boated’.