we like to start the engines on our challenger before we get to the ramp to be sure there is not going to be a problem,is there any problem doing this?
I do it too. I've watched WAY too many people launch a boat, and then tie up the ramp because their old POS won't start while it's sitting in the water.
Just don't run it for more than 15 or 20 seconds, and you will be fine.
Try it at home so you can fix things, just like Tony said - do not run so long to damage the hull seal. Taking the boat to the dock and leaving it on the trailer at the last ramp gives you a chance to run and test many things - observe any fuel/water/oil leaks and check the battery voltage when you lift the throttles a bit to get all the juices flowing - mark the oil tank to be sure it is drinking.