Whilst it was long ago I think that Townsend Thoresen ferries were the mot spectacular. Essentially they took an existing ferry (in fact I recall it was at least four, two at Dover and two at Portsmouth), cut off horizontally and lifted the whole accommodation block, stored it, Cut the hull vertically into two, electric glued in a new section, cut off the bow and renewed it, built a complete new vehicle deck onto the hull and then refitted the original accommodation block on too of larger ship hull. Have a look at before and after pictures at:
http://www.simplonpc.co.uk/Townsend-Thoresen3.html#anchor178427
Not particularly pretty, but economically very successful.