This should be in the non-seadoo section... but Yamaha's 800 engine has a very short life. The 701, and 760 yamaha were great engines. Anyway... find out if it's been rebuilt.
Ummmmmm......... 55 mph... yes, (60, no) if it's running properly, it should get you into the mid 50's on smooth water, and only you on it. The speedo will lie, and probably say you are getting close to 60.
I had a 2000 Yamaha XL760... and it was right around 50 mph. (and basically the same hull)