Bluedog,
There isn't any difference. Backing down a boat ramp down a ramp with a box is the same, whether it's powered by gas or diesel. A box is a box. Now, of course common sense applies to all situations. 99.99999999% of all boats will float off the trailer WAAAAAYYYYYYYYY before your bumper/lower body panel etc. of your class A or C ever comes near the water. Of course some of that depends on the boat ramp angle too.
But, for the most part, it might be easier to use a diesel pusher due to the fact that they have different turning radius's much of the time and, there for can react to adjustments in lining up the boat trailer with the ramp etc. easier. It also depends on the driving and handling characteristics of the driver too. If you're comfortable backing your fifth wheel in to tight, semi-tight camp spots, you'll be just fine backing your boat. Good luck. Have done it for years with both a C and a 34' A.
Scott