The sea is as blue as ever, the weather as fine, and, in this Provencal restaurant in a narrow alley in Cannes, the food tastier than ever. So why is Stephane, le patron, despondent? ''We have had a bad season,'' Stephane says, ''the worst in memory.'' The reason? ''The Americans didn't come this year,'' explains Stephane. ''That means a 30 percent fall in our revenue.'' The Tourist Office for the region of which Cannes is part estimates the loss of revenue due to an undeclared American ''boycott'' at around 18 percent, enough to make the difference between a good season...