I generally try to avoid large rivers, but there was a puddle on the apex of a corner at Silverstone t'other day and the front slid on it every time I hit the bugger!
The scientific answer is probably something to with with there being more weight through the contact patch per sq inch than a car, therefore less chance of a bike aquaplaning. However, I'm sure depth/pattern of tread, sideways shear forces and size of your cahoonas has more to do with whether you're going down or not.
In general Ducatis dislike being ridden into rivers (apart from the odd numptystroodle in off road form
)....
Cheers,
Ali