For all those who have Yamaha outboards and would like to change your own spark plugs the socket you need is:

18mm x 1/2" 6Pt Thin/Deep Socket (with and outside diameter of no bigger than 25mm)

I purchased mine from Bolts-R-Us in Geraldton and the part number is 29-1124018B and the make is Dual Action.

Hope this helps.