Underneath all the complicated fluid dynamical effects, a motorcycle can ride on water for much the same reason a speedboat does: it presents an upward slanted surface for water to bounce off of. For a speedboat, that surface is the hull; for a motorcycle, it's the bottom of the wheels, especially the front wheel. But wheels are complicated. It'll make things easier to start by examining the behavior of a speedboat-like slanted flat surface as it moves through water.