It's worth it for $500 only if you have the money, time, energy, and space to restore it to perfect or near perfect condition. Read enough boat restoration threads and you will find some seriously dedicated people, this hobby is quite addicting and can get expensive real fast.
My personal opinion on buying projects on the cheap is that if you go into it knowing that you will spend money to make it right, then it's a good deal. As soon as you try and cut corners because you have blown your budget, you will either be really mad at yourself for buying it in the first place, or you will jeopardize the safety of you and your passengers.
If I was buying this boat, I couldn't see spending less than at least $1500 on top of purchase price just to get it on the water with a reasonable amount of safety. A more realistic investment might be as high as $3500 on top of purchase price if you have to source out engine parts that may not be easily available or if you will be redoing the floors and/or transom.
Just my $.02 from buying and rebuilding 4 old Sea-Doo's and working on my Sunbird Restoration. By the way, you should check out my restoration thread in my signature as it's probably very close in construction to the Four Winns you are looking at.