Actually I have to say our car is pretty watertight. I took DaveW's suggestion and put 2 neoprene pads on top of the windscreen pillars and even driving under a virtual waterfall, or through a good 6" or more of water at 50kph (that was a surprise, wasn't it?) we don't get water in the car.
At 130 kph we get a slow drip coming over the top of the windscreen and under the hood rail but Morgan have given me the latest thicker neoprene to put in the bottom of the hood rail channel which should fix that