Let me try to elaborate. My last post sounded a bit "short" and I didn't mean it to.
The better, and really ONLY reason to start a jet boat off the hose is to make sure it'll start and idle. You cannot tune it to perfection off the hose. And here's why.
The engine is going to perform miuch differently under load than it will in the garage for one. Two, if you try and do this with the drive connected, then yes, you're going to do much more damage to the drive than you would gain. A disconnected driveline does nothing for you in tuning a boat, IMHO.
My point is this. If it starts, and you can get it to idle, get it in the water, under load. That's the ONLY way to tune it to it's potential
Just my .02, I'm sure some onw will chime in and prove me wrong.
Ray