I am British and I have an ESTA to enter Seattle, USA from the UK. Do I need a visa to travel to Victoria, Canada from Seattle by ferry?
Citizens of the United Kingdom do not need a visa to enter Canada.
If you arrive in Canada by air you would need to have an Electronic Travel Authorization, but since you're taking a ferry rather than a plane, carrying your British passport should be sufficient.
If you're not going back to the US from Canada after your visit, you may get some trouble because you may not be registered as having left the US when you leave by surface transport. Therefore when you enter the US later you may be suspected of having overstayed, and you should be traveling with documentation that you did in fact leave the US when you did. An entry stamp from Canada may do the trick, but other documentation of continuing a life outside the US will be helpful too.