Estimating average power draw is pretty much a guess.
The wiring between the amp and the battery needs to be sized large enough to handle the peak power, so essentially the maximum output of the amp. The alternator needs to be sized according to the average power draw which will be a lot lower.
If you don't have the volume cranked up then the amps will draw a lot less power. More volume = more power = more current.
With a 65 amp alternator you will find situations where the output is not enough, but they can be short duration periods that you can handle. For example: If you have the bilge blower on, a heater, cranking the tunes to be heard at wakeboard speeds and you are running three ballast pumps there is no way the alternator will keep up. Turn the bilge blower off, you don't need it while moving. The ballast pumps will only be on for a few minutes, your battery will last that long. Just pay attention to the volt meter on the dash: 13 volts or higher you are fine, below 13 volts and your battery isn't charging, below 12 volts and your battery is being drained.
The biggest drawback of an undersized alternator is that battery recharge time can become an issue. If you sit and listen to the stereo for a couple of hours you may need to put 100 amp-hours back into the batteries. If you only have 10 amps to spare out of the alternator it could take you all weekend to get the battery back to full charge.