Don't buy the warranty now. Yamaha will start soliciting you in 6 months. I purchased my extended warranty through them at 10 months in and they discounted it to get me to bite, emails after emails kept coming, it saved me hundreds. You get 1 year for your new purchase. You are free to purchase the YES warranty up to 1 year after purchase. Also, when you buy it online they can do it as a credit card or they can give you up to 4 monthly payments which is what I did. You have the option of 1-4 Years on the extended warranty and then you can decide. Perhaps you don't like boating, or you want to go to a bigger boat, no way to tell right now how long you should buy your warranty.
As far as pricing for the boat, you get what you get right now, otherwise someone else is right behind you to get it. Of course trying doesn't hurt. See if they will throw in a Coast Guard safety kit (PFD's, rope, anchor, air horn, etc.) and also see if they will offer you a discount on anything you purchase from them directly for a period of time. 10-20% off stuff. Ask if they will throw in a free 10 hour service, which is a very important maintenance that needs to be done.
Some people don't buy warranties and that is fine. I do on big things. I may never need them, but If the engine goes that is 10k plus. Highly unlikely but I personally am not willing to take the risk. If anything, if you decide to sell, a warranty makes it more attractive and you can get a slightly higher price for it potentially.