I've had my 2002 066 Yamaha 225 rebuilt. Seems to be running pretty well except I've lost about 8 mph (gps) of top end speed. Can the hot water and hot air temps affect top speed that much?
Also, when I try to see what it will maintain, the rpm' start to drop after about 20 seconds of getting to the top speed.
I've been told the drop in rpm could be a faulty O2 sensor. Could that also be affecting the speed of the boat? Tops out at about 5,300 to 5,400 rpm
Thanks

Yes, a bad 02 sensor will cause all sorts of gyrations with the fuel supply.
An O2 sensor sends a bias voltage to the ECM and the ECM uses this voltage data to process thru algorithms to determine how much unburned oxygen is in the exhaust gas If it is sending tainted bias voltage it may computing incorrect data showing an incorrect amount of oxygen in the exhaust. The amount of oxygen in the exhaust gas is used thru the programming to access the fuel map lookup tables to tell the injectors when and how long to fire at a given RPM. How long the injector sprays is called a duty cycle. Shorten the duty cycle and you cut fuel supply. Cut fuel supply and the engine loses power. Lots of things can cause an 02 sensor failure, one of them being water or moisture. Other things are fuel additives that the sensor's platinum coated zirconia ceramic element does not like.

Thank you for the information. Looks like this will be my next effort to bring this motor back into good working order. I have been reading about using O2 sensors that are not the Yamaha oringinal factory replacement. Hesitant to go that way just to save $200 bucks when I have so much invested to this point.

Just be sure you get the correct one for your engine. There are 3 types of o2 sensors, narrow band, wide band and broad band. Most likely yours is a heated wide band sensor with white heater wire. Using the wrong one will not turn out well. Lots of good info here http://www.thehulltruth.com/boating-forum/394510-replaced-bad-o2-sensor-ox66.html