--You`ll need an esta (if travelling under the visa waiver scheme) for the States though.--
True.
But I'm wondering about getting back into the US at the end of the cruise. Three possibilities:
if it's a US cruise ship maybe you are treated as if you never left the US
if the cruise company is a participating carrier (I think the term is) then ESTA is fine
if the cruise company is not a participating carrier then you might need a visa.
Are they in this list:
http://www.immihelp.com/visa-waiver-program/visa-waiver-program-approved-carriers.pdf