Throwing some info:
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http://www.theweatherprediction.com/habyhints/216/
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http://www.windows2universe.org/earth/W ... nsity.html
So normal water has a density of 1000kg/m^3; air at sea level, relative comparable to Suzuka since it's pretty close to sea level I believe, has only 1.275kg/m^3. Density of water increases slightly when talking about ocean water.
Of course, the cars didn't actually run in an aquarium, so the overall density, air plus rain, is a lot lower, but local density should be quite high near the track surface (that's where you'll get closest to an aquarium). The main thing thing to note is that tyre drag increases by quite a margin due water displacement. A driver slipstreaming isn't facing that issue, so has a lot less drag then the guy in front.
Call it a natural DIS: Drag Increasing System. For the guy in front.