Not unusual enquiry - have a search for some of the container ship lists that take passengers - most offer passenger accommodation, some of which is really posh - owners cabins etc, most have a pool and dead good food too - as well as taking you to places to stop off at en-route.
Downside is you could have a shipping container in front of the cabin windows......and you are on the sea voyage for more than a couple of weeks, so have a good library.
Couple of links to see:
http://www.cargoshipvoyages.co.uk
http://members.aol.com/CruiseAZ/freighters.htm - this is the site you get taken to from
http://www.cruisepeople.co.uk . The freighter site has a stack of lists of freight companies that you can approach for passenger avilability - the sailing is about 40 days UK - NZ, and you could opt to go via Suez or Panama and Tahiti - have a look and enjoy..!
There are UK only coasters you can 'try' before you commit to a long voyage by the way - eg take a trip Liverpool to Orkney (as on the first link above) and see if you like the facilities/experience - might be a good short break anyway, and would give a taste if you haven't been at sea for a long period anyway.
Or holiday at home - train and bike tour? Walking tour ? All very green....