Thinking about buying a 2017 Skeeter with 50 hours total on engine. Original motor. One owner. All service done at the dealership where it was bought. Exterior of boat is in excellent shape. Original warranty on motor is thru March 2020. So I basically have one month left of coverage. Can buy an extended Yamaha warranty for $1500. Yamaha 225SHO. Is there a diagnostic test I can ask the service department to run before I buy the boat? Is an extended warrant worth the $$?

You can shop the warranty. The prices on the exact same warranty can be all over the place.
And yes, the money spent on a thorough diagnostics test is cheap insurance.

By all means if you can get the warranty for $1,500 then buy it. I think they have a 3 year available also? I bought the 3 year for $2,200, cheap piece of mine because my sho 250 cracked a head just before the regular warranty ran out. Didn't cost me a dime and put a complete new powerhead on, but if it had been out of warranty, would have been about $8k for everything. I later sold the boat and only reason the person bought it was because it still had 1.5 years of warranty. My replacement boat doesn't have warranty and I worry about it all the time, lol!

Buy the WARRANTY. I had 240 hrs on my 250 SHO and also got a cracked head. Purchased new in 2011 with 6 year warranty until 2017. Motor went out in November 2019. Repairs were quoted at $12,800 with a 1 year warranty. Yamaha wouldn't help, so I now have a 250 Merc 4s on it. Couldn't see spending over 50% for repairs and there has been TOO many reports of cracked heads in 2009-2013. Hopefully, the Merc will at least give me 500 hours on it. :realmad