Rubber Boats

I am in the process of designing a France 1940 game based on the German attack on Sedan, France. A big part of the game will be a river crossing by German infantry in rubber boats. Are there any rules or suggestions for this type of assault i.e movement in water, transport capacity etc

Not that I know of, but you could treat movement on water as difficult terrain with random movement by d6 to simulate changing currents. The Overlord supplement has these rules for landing craft:
Disembarking: within 1” of shore line. Infantry figures are placed within d6”. Vehicles placed within 6” of ramp, no more than 3 vehicles per turn may exit any one landing craft.
Sinking: Take 2 d6 hits. Any surviving infantry aboard must swim ashore d6”: if a 1 or 2 is rolled, take d3 casualties and check morale. Any special equipment or heavy weapons are lost (become riflemen)