I think Yamaha is a good product, I have a 1985 25 hp yamaha on my fishing boat that I have had since new and the only thing I have done to it is change spark plugs and clean the carbs a few times, it has seen heavy use, when I was a kid I would run it all day every weekend, it was submerged once for over 24 hours when someone stole it 18 years ago and it still runs like a top.
You probably don't need the extended warranty but if something goes wrong you'll be kicking yourself for not buying it, it's kind of a gamble.
Off topic here a little but still warranty related- I usually buy a new Polaris snowmobile every two years- always buying the extended warranty, I had no problems with 4 sleds so on the last one I decided to save the $850.00 for the extended plan- of course two weeks after my warranty was over the engine blew up- cost me over $3000.00
Sorry for the the long winded post, just trying to give you a few different way's to look at it, again, when talking about a Yamaha O/B in my opinion they are bullet proof.