Got a YSB12 which is v similar. There should be an anode fitted in the top of the engine block. Depending on version this is either central (should be a square nut!) or on the side of the block above the cylinder head.
Failing this try EP Barrus the Yanmar distributors who can usually help or alternatively Marine Power in bursledon (http://www.yanmar.co.uk/) or Sandy at Troon Marine Services both of whom are helpful and can supply the spares.
If you are trying to keep an engine of this age going (mines a mere 21) you do need the parts list (essential!) and overhaul manual (useful) which any of the above can supply.
Two areas to keep an eye on - salt water eventually destroys the original cast iron exhaust goose necks, the new ones are bronze. The only other major problem i've had is the starter solenoid needed rewinding the Yanmar part is £100+ but marine power were able to provide a replacement a lot cheaper.
I always start with the decompression lever on to reduce wear on the starter - let the engine get to speed and drop the decompression.